A team of 30 detectives from Scotland Yard will be assigned to the search for missing Madeleine McCann in an investigation which could cost millions of pounds, The Daily Telegraph has learnt.

Kate McCann holding her missing daughter, Madeleine's favourite toy, Cuddle Cat.  Photo: JOHN TAYLOR

Some of the team will be officers who were due to retire or take voluntary redundancy, a police source said.

The details came as Sir Paul Stephenson, the Commissioner of the Metropolitan Police, defended Scotland Yard’s decision to take up the case, insisting that Madeleine could still be found alive.

She went missing from her family’s holiday flat in Praia da Luz in the Algarve on May 3, 2007, shortly before her fourth birthday.

Portuguese police, helped by officers from Leicestershire Police, carried out an extensive investigation into her disappearance but the official inquiry was formally suspended in July 2008.

Since then no police force has been actively looking for the child.

But last week, it was announced that Scotland Yard would reopen the search, a move that led to criticism that officers’ valuable time would be diverted away from other cases. A police source said that the team of 30 Scotland Yard officers would in part be drawn from those who were due to leave the force through redundancy or retirement.

One of the major difficulties — and expenses — that detectives will face is that much of the material in the investigation will require translation from Portuguese, the source said.

However, the commissioner insisted “there is always a chance” his detectives would find Madeleine alive.

“When you receive a request, supported by the Prime Minister, from the Home Secretary, you take that very seriously,” he told LBC radio. “This is not unique. On balance, I think it was the right thing to do.

“We do review these sort of cases and I think there may be some benefit here.”

The review, which will be funded by the Home Office, has already been described as “ludicrous” and prompted claims that the intervention could undermine the independence of the force.

Last week, Lord Harris, a member of the Metropolitan Police Authority, said: “Whilst no one doubts the desirability of doing what can sensibly be done to find out what has happened to Madeleine McCann, I can imagine that the senior leadership of the Metropolitan Police are not exactly happy about this.

“It again embroils their officers in a high-profile investigation, where the chances of success are unclear, and which will divert limited investigative resources away from other matters.”

Scotland Yard will review the Portuguese police files on the case. The review will be led by Detective Chief Inspector Andy Redwood, from the Homicide and Serious Crime Command.

The announcement of the Met’s involvement followed criticism of David Cameron from Madeleine’s parents, Kate and Gerry McCann, who said the Prime Minister had shown insufficient commitment to children.

The official Portuguese inquiry into Madeleine’s disappearance was formally suspended in July 2008, although private detectives employed by the McCanns continued the search.

Mr Cameron personally wrote to the McCanns to confirm the reopening of the investigation.

The couple have welcomed the move as “a step in the right direction”.