THE official hunt for missing Madeleine McCann was dramatically back on tonight after the PM acted on her parents' heartfelt plea for help, The Sun can reveal.

Moved David Cameron ordered the Met Police to reopen all the Madeleine files for a full review of all the evidence ever gathered on her.

If the probe finds any fresh leads over how the little girl disappeared four years ago, a full-blown police investigation will then begin.

The PM's decisive move comes as more than 20,000 Sun readers signed our online petition today calling for an independent and transparent review of the Maddie files.

Met's Commissioner Sir Paul Stephenson was tonight pulling together a "sizeable" elite team of detectives for the job.

They will go over every jot of evidence gathered by cops in the past "with a fine-tooth comb", a senior government official insisted.

Kate and Gerry McCann told The Sun: "We welcome the Government's response. 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 the review process.

"We would also specifically like to thank The Sun, News International and the general public for supporting the campaign to find Madeleine in the way that they have."

Madeleine was abducted from a holiday apartment in Praia da Luz, Portugal, on May 3, 2007. Today is her eighth birthday.

Her parents, Kate and Gerry McCann, made their appeal to the nation's leader in an open letter to him published in today's Sun. Kate's book on her daughter - titled simply Madeleine - has been serialised all week in The Sun.

Clearly touched by the McCanns' painfully emotional words, the Premier held urgent talks with Home Secretary Theresa May today on what they could do. Father-of-three Mr Cameron met the McCanns 18 months ago when he was Opposition Leader and has followed their ordeal closely.

The Sun can also reveal that Home Office officials have been secretly working on reopening the hunt for Maddie - for the last nine months.

Kate, 43, and Gerry, 42, spent an hour with Mrs May in August last year to ask for her help with their fruitless search.

Mrs May told The Sun tonight: "We all want to see this beautiful little girl returned to her parents.

"That's why we have been doing everything we can behind the scenes in the search for Madeleine.

"Although it might not always be in the public eye, the British authorities have never given up on their work to find Madeleine."

The Home Secretary has also agreed to foot what is likely to be a substantial bill for the case review from her department's budget.

Leicestershire police, as the McCanns' local force, were the 'liaison force' responsible for collating all investigation work done by cops all over the UK.

The British police, at the request of the Portuguese cops, carried out interviews with UK witnesses and also collated sightings reported to police in this country.

All the information was then sent onto Portuguese police.

There was never a British investigation into Madeleine's disappearance as her abduction happened in Portugal.

It was one of the reasons why the Leicester police force constantly refused to comment on the case, even to Kate and Gerry, simply stating: "This is a Portuguese investigation."

The case review tonight won cross party backing in Westminster.

Shadow Home Secretary Yvonne Cooper gave it her full backing, saying: "We fully back Kate and Gerry McCann's request for information in Madeleine's case to be reviewed."

Madeleine's parents have been campaigning for a review of her case for several years.

The couple feel it is 'crucial' to finding their daughter - fearing vital leads, sightings and other clues are hidden in Portuguese police files.

They are desperately hoping that a fresh look at the files, similar to a criminal 'cold case' review, may discover something that was overlooked in the original investigation.

Gerry said: "The files are not held on a single database anywhere. By putting them all together we could discover that vital piece of the jigsaw."

The couples' fears, of vital clues being missed, seemed to be borne out when The Sun obtained files of sightings and other reports which had been collated by the Portuguese and British police AFTER Madeleine's case was shelved by them in July 2008.

As we reported at the time most of the reports were stamped 'No action' by the Portuguese detective in charge of collecting them and then left to gather dust in a filing cabinet.

Speaking at the time Kate described the revelation as "heartbreaking" adding "it is just not fair to Madeleine".

All the files relating to Madeleine's case are currently held by the Portuguese Judiciary at police headquarters in Portimao, Portugal.

The files include witness statements and details of sightings.

A month after the Portuguese police investigation into Madeleine's disappearance was shelved in July 2008 over 30,000 pages of files were released in Portugual.

But they were only released to people who had made a court application for the right to see the police files - the McCanns included.

Lawyers on behalf of the couple had to journey to Portugal to make the application on their behalf.

But the released files did not include all the documents held by Portuguese cops. Some of those were held back, with police saying they could not be made public.

Two years later files which contained evidence and sightings collected AFTER Madeleine's case was effectively closed in 2008 were also released.

But again not all the documents were made available.

The McCanns have collected all the information that has become available and their private investigation team have put it together as best they can.

But they are hampered by the fact that all the documents and pieces of evidence have still not been handed to them.

The investigators, and Kate and Gerry themselves, have spent the last three years ploughing through the files and trying to follow up as many leads as they can - but as Gerry admitted recently they are a 'small team' and only able to do so much.

As it stands it would take them years and years to effectively investigate all the clues and sightings in the files - and the money from the Find Madeleine fund which pays for the team would run out well before they could do that.