Police sources have played down the significance of an intelligence report suggesting Madeleine McCann might have been kidnapped by a paedophile ring.
The Metropolitan Police got a tip-off from an informant earlier this year and passed it onto Portuguese counterparts.
That line of inquiry was one of thousands of leads detailed in Portuguese files on the disappearance.
It will be followed up by the McCanns' private investigators who are looking at 11,000 pages of evidence.
The Metropolitan Police received the tip-off in March this year - 10 months after Madeleine's disappearance, aged three, in May 2007 while on holiday in the Algarve.
Lack of detail
An anonymous caller claimed that a Belgian paedophile ring had placed an order for a young girl - and that Madeleine was kidnapped in Portugal three days later after a paedophile had 'approved' a photograph taken secretly during the child's holiday.
That information was passed on to Portuguese investigators who, in turn, sought the help of the Belgian police.
Faxes and e-mails in the police files suggest that officers in all three countries found it difficult to pursue the lead because of a lack of detailed information.
Metropolitan Police sources have played down the line of inquiry, stressing that the intelligence was second- or even third-hand and impossible to corroborate.
A spokesman for the Belgian police told the BBC they had received dozens of calls about Madeleine - with varying degrees of credibility.
He added there was not a "shred of evidence" that the country was the base for an international paedophile ring.
Family spokesman Clarence Mitchell said he could not comment on sensitive case files which were in the hands of the McCanns' private investigators.
Those files include a 14-volume annexe of reported sightings of Madeleine - from Spain to Serbia, Mexico to Indonesia.
But Portuguese police concluded most were "of little substance and purely speculative".
On 21 July Portuguese prosecutors announced they were shelving the case, although it could be re-opened if credible new evidence came to light.
At the same time the McCanns and Algarve resident Robert Murat were told they were no longer "arguidos", or formal suspects, in the investigation.
They have always strongly denied having had any involvement in what happened to Madeleine.