Scotland Yard is to take a fresh look at the evidence in the case of missing Madeleine McCann.
The move comes after Kate and Gerry McCann made an impassioned appeal for David Cameron to help them revive the search for their daughter, who vanished Portugal in 2007 shortly before her fourth birthday.
The Prime Minister wrote to the McCanns to pledge action, having met the couple 18 months ago while opposition leader.
Home Secretary Theresa May said the Metropolitan Police would use its “particular expertise” to review the case and that the Home Office would provide “the necessary financial support”.
In a letter published in the Sun, Mr Cameron said: “I have asked the Home Secretary to look into what more the Government could do to help find Madeleine.
“She will be writing to you today, setting out new action involving the Metropolitan Police Service which we hope will boost efforts in the search for Madeleine.”
He told the couple: “Your ordeal is every parent’s worst nightmare and my heart goes out to you both.
“I simply cannot imagine the pain you must have experienced over these four agonising years, and the strength and determination you have both shown throughout is remarkable.”
Mrs May said: “I am pleased to announce that the Prime Minister and I have agreed with the Metropolitan Police Commissioner that the Met will now be using its particular expertise to review the case.
“The Met have skills, techniques and know-how which we hope can bring a new perspective to the case.”
She said officials had continued to work “behind the scenes” in the search for Madeleine, adding: “Although it might not always be in the public eye, the British authorities have never given up on their work to find Madeleine.”
A Home Office spokesman added: “Although she disappeared in Portugal, and the Portuguese retain the lead responsibility in the case, law enforcement agencies here have continued to follow up leads and pass information to the Portuguese authorities as appropriate.”
He added that detail of what the Met’s involvement would entail “will be a matter of operational judgment and it would not be appropriate to discuss at this stage”.
Portuguese detectives helped by British officers from Leicestershire Police carried out a massive investigation into Madeleine’s disappearance from her family’s holiday flat in Praia da Luz in the Algarve on May 3 2007.
But the official inquiry was formally shelved in July 2008 and since then no police force has been actively looking for the missing child.
Former home secretary Alan Johnson commissioned a scoping exercise by the Child Exploitation and Online Protection (Ceop) centre to look at the feasibility of carrying out a review of the case.
This was completed in March last year, but Mrs May refused to let him and his wife see it because it was “sensitive”, Mr McCann said during a press conference yesterday.
Prior to the Home Office’s announcement, he called on Mr Cameron to offer Portuguese caretaker prime minister Jose Socrates the services of the Met to review the investigation looking for unpursued lines of inquiry.
He added that he would be “delighted” if FBI officers could also form part of the cold case team.
Later, in a statement, the couple welcomed the “review” and thanked the Government for committing such a significant resource as the Met to the search for their daughter.
“We welcome the Government’s response,” they said.
“This is clearly a step in the right direction.
“The expertise of the Metropolitan Police is renowned and we are reassured by our Government’s commitment to the search for Madeleine.
“We would like to thank Mr Cameron and the Home Secretary for committing such a significant resource as the Metropolitan Police to begin this review process.
“We would also like to thank the general public for the way in which they have continued to support our campaign to find Madeleine.”
Their spokesman Clarence Mitchell told Sky News: “They have had lots of reassurances but this is the first time there seems to have been definite movement towards achieving the end result they want.
“That is simply to make sure that nothing has been missed, that there is not a vital piece of information sitting in Portugal, or in Britain, or in the private files, that linked up will suddenly unlock this.”
Shadow home secretary Yvette Cooper told the Sun: “We fully back the McCanns’ request for information on Madeleine’s case to be reviewed.”
The couple hope that Mrs McCann’s book, entitled simply Madeleine, will revive public support and boost their dwindling fund for private detectives to search for their daughter.
It was already a best-seller based on advance sales, having topped Amazon.co.uk’s book chart for several days.
The 384-page book, which is dedicated to the McCanns’ three children, Madeleine and six-year-old twins Sean and Amelie, includes intriguing insights into the couple’s surreal existence in Portugal in the summer of 2007.
Mrs McCann discloses that Cherie Blair phoned her five days after Madeleine’s disappearance and warned her: “Whatever happens, your life will never be the same again.”
The book is being translated into Portuguese, and Mr McCann said he hoped it would be widely read in Portugal to help clear up some long-standing cultural misunderstandings about how he and his wife acted.