Team McCann is now hiring a fifth group of private investigators to help with the search.

Mr Kennedy, who has five children with his wife Christine, lives in Wilmslow, Cheshire, and made his money through double-glazing firm Everest. He had not met the McCanns before offering his help but now speaks to them almost daily.

A source said: 'Mr Kennedy got involved after seeing Kate on television after she became an arguido. He felt compelled to do something. A fifth company is going to be hired.

'Everything that can be done is being done. But it is like looking for a grain of sand on a sea shore. The team are just desperate for leads now.'

The couple's lawyers are still sifting through 13,000 pages of Portuguese police files, released last month. One team of investigators believed they had found Madeleine in Morocco last year ' but it turned out to be an American girl being kept against her will in a dispute between her parents.

'It was desperately disappointing when it turned out not to be Madeleine,' said a source. 'It was a low point for everyone concerned.'

Madeleine's Fund is already paying investigators Metodo 3, Control Risks, Oakley International and an unnamed fourth company.

Clarence Mitchell, the McCanns' spokesman, said: 'We will leave no stone unturned in finding Madeleine.'