A MISSING blue tennis bag could hold the key to Madeleine McCann's disappearance, a TV documentary will claim tonight.

The bag - big enough to carry a small child - belonged to Maddie's dad Gerry, the programme says.

It is claimed Portuguese cops have been hunting for the holdall, which disappeared at the same time as the four-year-old girl.

The bag emerges as a potentially vital new clue in a Sky News investigation entitled The Madeleine McCann Mystery.

Yet, bizarrely, the McCanns' spokesman Clarence Mitchell yesterday denied it existed - saying Gerry did not own such a bag and never had done.

Sky's crime reporter Martin Brunt stood by the documentary's claim last night.

Former detective Tony Rogers, one of Britain's most experienced police officers, will be seen saying: "If it's a bag of a size that could be taking away from the flat a child, that would be of great interest to the investigating officer."

The development came as Gerry and wife Kate, both 39-year-old doctors, faced their first Christmas without Maddie - who vanished on May 3 in the Portuguese resort of Praia da Luz.

Yesterday, a family friend demanded that the "fundamentally flawed" case against them be DROPPED immediately by Portuguese police. The call came over fresh doubts about the validity of DNA tests.

It is thought the Maddie probe has involved the same process of so-called "low copy number" DNA testing used in the Omagh bombing case - thrown out of court after a judge ruled it "potentially unreliable".

Tonight's documentary also claims Portuguese detectives have failed to find and quiz a British paedophile with links to the Algarve. He was among 52 British sex offenders with similar connections whose details were given to police by UK cops.

It is not known if he was the man who, in September, was at the centre of the hunt for the tot after witnesses picked him out from photos of 100 perverts.

Nor can he be definitely linked to another suspect - the 32-year-old son of expat Brits who is said to have "groomed" and sexually assaulted a girl of 12 in the resort

two years ago. That man is thought to be hiding somewhere in the UK.

Brunt told The Sun a British source close to the probe said Portuguese cops had "worked very hard" to find the 52 and rule them out. But he added: "There is still one man they are concerned about."

The documentary quotes top forensics consultant Professor David Barclay. He said Portuguese police had "looked at" paedophiles in the Praia da Luz area. He added: "They didn't check up on other people staying in the apartments."

The new call for the McCanns to be cleared followed the storm over low copy number DNA testing - a technique allowing results to be interpreted from samples containing just a few cells.

The pal said: "If the leaks from the police to the Press are true that low copy number testing is involved then we would demand the reliability of these tests is absolutely pinned down before it is used in cases such as ours.

"It is ridiculous to form a case against someone using evidence that is fundamentally flawed."

Gerry and Kate went to church in Rothley, Leics, yesterday with their two-year-old twins Sean and Amelie.