It is understood a mysterious call claiming to know the whereabouts of the four-year-old came from a mobile phone registered in Argentina.

The "credible" call was considered so potentially significant that the McCanns halted their search of Europe to help police investigate.

They delayed their flight from Berlin to Amsterdam by three hours and plans were drawn up to divert to the UK.

Silent prayer: Kate McCann grips Cuddle Cat after today's balloon release

It was thought the McCanns might need to return to Britain to talk to specialist advisers about the call.

The call from the pay-as-you-go phone came from a man who wanted to speak directly to the McCanns, according to Spanish police sources.

He did not reveal his identity or nationality, but the phone was soon linked to the South American country.

All efforts to re-establish contact with the caller failed on Wednesday and the couple carried on with their journey around Europe.

A British police source said: "The importance of this line of inquiry is still being assessed and attempts to re-establish contact are continuing."

Although Spanish officials denied they had received the call, a Guardia Civil source told the Portuguese paper Correio de Manha: "Only time will tell if this call gives help or not to the case."

Spanish newspaper El Mundo reported that a man matching the description released by Portuguese police two weeks ago was seen in a bar in Seville a week before Madeleine's abduction.

It claimed the man was working on the instruction of others and told fellow drinkers he was going to the Algarve.

Hope: Kate and Gerry McCann release yellow balloons today as the hunt for Madeleine goes on

The latest development comes on the day Portuguese police were forced to defend their reputation amid allegations that they were enjoying boozy lunches while the search for Madeleine continued. Armed police officers were also criticised by Madeleine's aunt for preventing her from putting up posters of the little girl at Lisbon Airport.

Senior police officers involved in the investigation were seen laughing and joking as images of the missing four-year-old and her desperate parents appeared on a restaurant TV screen.

It happened at a lunch lasting nearly two hours as Kate and Gerry McCann were away campaigning in Europe.

They laughed and cracked jokes as they enjoyed a meal washed down with wine and whisky - as footage of the couple played in the background.

Afterwards, they left a table littered with empty glasses - and went back to work.

Yesterday Policia Judiciara (PJ) spokesman Olegario Sousa, one of the officers spotted having lunch, said it was up to the individual to decide what he or she ate and drank.

Asked if it was acceptable for police to drink alcohol in their lunch break he said: "I don't know, it is very, very sad but a person's free time is for lunch. That is normal to do.

"The persons are in charge in the day, they are working in the day but they must eat and drink - it is normal.

"I drink what I want to drink when I can drink."

When it was put to him that he had been seen drinking, he said: "Have you seen anyone drunk? Have you seen any action deterred by that?"

Mr Sousa and Goncalo Amaral, head of the regional PJ, were spotted as Kate and Gerry McCann travelled to Berlin and Amsterdam to appeal for more information about their missing daughter.

In Portimao, a town near where the four-year-old was snatched 35 days ago, a diner at fish restaurant Carvi said he recognised the police officials.

"I knew who they were because Mr Sousa has been all over the TV and in the papers," he said.

The diner watched as officers enjoyed the lunch, which took place a short walk from the police station less than 24 hours after Kate and Gerry McCann were told that everything possible was being done to find their little girl.

Then - in what looked like becoming the first arrest in this case after nearly five weeks, a photographer trying to take a picture of them emerging from the restaurant was detained, held for four hours, fingerprinted, interviewed, and had his camera confiscated. He has now been formally named as an 'Arguido' - the same status as the chief suspect in Madeleine's disappearance, Robert Murat.

A 'credible' caller claiming to know where Madeleine is gave enough detail for the McCanns to put a brief hold on their trip to Amsterdam

On Tuesday, two groups went to two separate restaurants. The bigger party did not begin to leave for an hour and three-quarters. The smaller party had a 50-euro meal of fish and wine and shared jokes between what appeared to be discussion about police business.

On Wednesday, the party included senior figures from police headquarters at Portimao, where the investigation is based. One of them was Ch Insp Olegario Sousa, the public face of the inquiry, who appears on TV at press conferences. Another was Goncalo Amaral, number three in the investigation and a well-known figure in major police operations.

At 12.50pm the two men strolled across a sun-drenched square to Carvi restaurant, a regular haunt that specialises in fresh seafood and lobster straight from the tank. Inside, they formed a table of four with two other officers.

The diner said: 'They asked for the Portuguese TV news to be switched on and sat at the table watching it. It must have been about 2pm. Madeleine's parents had given a press conference in Berlin and they came on the screen.'

At that Berlin conference, Gerry McCann had made it clear he was confident police were doing all they could to find Madeleine. During a live broadcast that morning he had said: 'We have had no doubts about the desire of the police to find Madeleine. We have witnessed their efforts first hand and they're working harder than Kate and I.'

The diner added: 'The police were laughing and joking among themselves while it was on. They seemed to be sharing some sort of joke. Whatever it was, I thought that laughing like that in public was in really poor taste.

'They had a bottle of chilled wine with the meal but they had a bottle of whisky on the table after the main course as well. I was pretty shocked to see they were drinking whisky at lunchtime. The bottle was passing between them for about half an hour.

'Someone on another table seemed to know them and joked about them having two-hour lunches and knocking back Johnnie Walker Black. He said they would get themselves in the papers.

'There was a guy in a red shirt holding court about Portuguese law. They were discussing a change in the law being planned for Arguidos.' (Portuguese for suspect).

Two of the party left, then Ch Insp Sousa left on his own, leaving a colleague behind.

'I got the impression they went there regularly - they were very friendly with the waiter. I don't know what time they came in but I was there for a good 90 minutes and when I left, one of them was still slumped back in his chair in the corner with the whisky bottle in front of him. He was a big sweaty guy and he was sagging into the chair. The table was littered with empty glasses.

'There was some sort of commotion and I heard someone shout out. They swore and said something about the 'Paparazzi Ingles' (English Paparazzi) hiding behind the door.'

One officer had insisted privately the Madeleine officers had been working 'punishing hours', sometimes sleeping overnight at the station in the early days of the inquiry.

Philomena McCann, Madeleine's aunt, said such behaviour would not be acceptable in the UK: "If it were detectives from Scotland Yard there would be absolute uproar.

"But we have to let them to get on with their work because that's all we have to rely on.

"It is a different country and we have to accept the way that they do things and that it is a different culture where they have lunches and siestas but we hope the work is made up at other times."

She then told how armed police officers stopped her putting up posters of the little girl at Lisbon airport.

She and another relative were travelling from the Algarve to the holy shrine at Fatima when they made a diversion to the airport.

Kate McCann had noticed there were no pictures up when she passed through on her way to Madrid.

"She was so upset to think there were so many tourists coming in and out and nothing there to remind people of Madeleine," said Ms McCann.

"She asked me to make a detour on the way. I was given permission to put the posters up by a woman on the information desk.

"But straightaway we were swooped on by two armed police officers. I was with a relative who was bodily manhandled by them.

"We went back to the information desk and there was a big row between the woman and the police."

Ms McCann said the director of the airport Dr Francisco Severino told them they could fax a request which would be considered.

"It would be fair to say we were unimpressed by their unhelpful attitude," she said. "We were very badly treated.

"It seemed clear they didn't want the negativity affecting tourism but I think they are doing the wrong thing.

"Surely if people think the police and the authorities are doing everything they can to find Madeleine other families visiting Portugal would feel more secure."

Ms McCann said she had asked junior Justice Minister Baroness Ashton to put pressure on to change their policy.

The McCanns are back in Portugal today ahead of a trip to Morocco, where there has been a reported sighting of Madeleine.

In Praia da Luz today, the couple watched as 1,000 yellow balloons calling for information about Madeleine were released into the air.

Meanwhile in Praia da Luz, the Algarve resort from which Madeleine vanished on May 3, police removed their 'do not cross' tape from the McCanns' holiday apartment and withdrew all police presence exceprt for one uniformed officer outside. Alipio Ribiero, national director of the Judicial Police, said: 'The Judicial Police are seriously investigating this case. It could take time but we continue in the Algarve, even if our presence is not noticed.'

The exhausted couple had their hopes dramatically raised that their daughter was still alive yesterday - only to see them dashed.

The couple's planned flight to Amsterdam on Wednesday night was held for three hours in Berlin after what appeared to be a crucial breakthrough.

They were told that a "credible call" had been received by Spanish police from a man suggesting he knew where Madeleine was and saying that he wanted to talk to the McCanns.

The call was reportedly traced to an unregistered pay-as-you-go phone outside Europe.

The caller did not disclose his identity, but the information supplied was apparently so specific that British police liaising with the Portuguese inquiry felt it necessary to tell the McCanns immediately.

The couple were advised that the mystery source might try to make contact, and that they should delay their flight in case he called when they were in the air.

As frantic efforts were made to re-establish contact with the caller the McCanns were whisked off the flight, waiting anxiously for nearly three hours at the British Embassy in Berlin. The man never called back.

Journalists on the plane were told that the crew had been asked to draw up a new flight plan involving a possible switch of destination from Amsterdam to East Midlands Airport, close to the McCanns' Leicestershire home.

But at 7.30pm the flight was cleared to continue to Amsterdam, where the McCanns pressed ahead with their European campaign to keep their daughter in the public mind.

Soon after they touched down, it appeared that the call was a hoax, or was no longer being treated with any urgency.

Spanish police categorically denied that they had received such a call, as did the Spanish Interior Ministry.

It was an illustration of the kind of distractions the McCanns are having to endure in their relentless search for information about Madeleine, who vanished more than a month ago during the family's holiday in Portugal.

Another followed soon afterwards when a Spanish newspaper quoted an "investigative journalist" claiming he knew the identity of Madeleine's abductor, and suggesting she had been stolen to order by a paedophile ring.

Last night, however, there was no indication that police were investigating the claim.