Private detectives working for the girl's parents are urgently following up the report from an informant disclosed in Portuguese police files.
The tip-off was recorded by an intelligence officer working for Scotland Yard's CO14 clubs and vice unit.
But sources said the information was second or even third-hand and impossible to corroborate with forces in the UK or abroad.
It is believed to have been one of dozens of similar reports passed to the unit in the wake of Madeleine's disappearance.
The informant said a photograph of the child on holiday in Portugal was taken and passed to a "purchaser" in Belgium days before she vanished.
The confidential report read: "Intelligence suggests that a paedophile ring in Belgium made an order for a young girl three days before Madeleine McCann was taken.
"Somebody connected to this group saw Maddie, took a photograph of her and sent it to Belgium.
"The purchaser agreed that the girl was suitable and Maddie was taken."
Portuguese police pursued the lead with Interpol, which gathered further reports from Belgium, the UK, Finland and Germany.
But detective Paulo Rebelo, head of the Madeleine inquiry, ruled that all but the German intelligence showed "lack of credibility".
As a result, the Scotland Yard tip-off was added to the massive file of evidence considered during the hunt for Madeleine.
A Metropolitan Police spokeswoman said everything possible was done to follow up the snippet of information.
She said: "Unsubstantiated information was received by CO14 relating to the disappearance of Madeline McCann.
"This was passed onto Leicestershire Police on 4 March 2008.
"The information was further discussed with Leicestershire Police verbally and all possible lines of inquiry were conducted.
"Leicestershire Police passed the information on to the Portuguese Authorities."
Belgium is less than 100 miles from Amsterdam where there have been two possible sightings of the missing youngster.
Shop assistant Anna Stam, 41, spoke to a little girl called "Maddy" who said she had been taken from her mother while on holiday in May last year.
Hannie Wiechmann, 71, called police after seeing a young child she believed to be the missing girl in the second week of May last year.
She told the Dutch newspaper Metro she "just knew" it was Madeleine because of her eyes and her "unequally-cut fringe" and dyed hair.
Police also investigated a sighting of a girl who looked like the missing child at a service station near the town of Tongeren on Dutch border last August.
A friend of Kate and Gerry McCann, both 40, from Rothley, Leicestershire, said the couple's detectives were taking the Belgian link very seriously.
"Trafficking into Belgium forms a very strong part of their investigations, as does trafficking into North Africa," the friend said.
"There is definitive hard evidence that this is very much alive and happening, and they have looked into the fact that someone was targeting children and they may well have been stealing to order."
McCann family spokesman Clarence Mitchell refused to comment on "sensitive" information found in the Portuguese police case files.
He said: "Clearly any information of that nature - Kate and Gerry would hope that the Portuguese police in conjunction with Interpol have acted to the absolute best of their ability in following this up.
"The private investigators, as with all the other sightings and all the other information, will be pursuing this line as an absolute priority to establish if it has been fully investigated and properly ruled out.
"They have got some of the information already from their lawyers and investigators and they are waiting to hear from them what is legitimate, what is promising and what is not.
"They are frustrated by the delays and the mistakes that were made.
"They have learned an awful lot about this kind of thing, and God forbid she has fallen foul of any of these types."
Belgium has been rocked by several appalling paedophile cases, including that of unemployed electrician Marc Dutroux.
The 47-year-old was jailed for life four years ago after being convicted of kidnapping, raping and murdering young girls.
In one interview Dutroux claimed he was part of a wider paedophile ring but police did not want to pursue it.
Dutch police refused to comment, referring inquiries to the Portuguese authorities.
Madeleine was nearly four when she vanished from her family's holiday flat in the Algarve resort of Praia da Luz on 3 May last year.
On 21 July Portuguese prosecutors announced they were shelving the case, although it can be reopened if credible new evidence comes 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.
Responding to the Metropolitan Police intelligence, a spokesman for Belgium's Federal Police said: "At this point we do not have any information such as that quoted in press reports.
"Our police, our missing person team, paedophile unit, are not aware of this information. We are checking with the British police.
"We find it all a bit strange. We are not aware and we have never found a paedophile network in Belgium that could order, sell or buy children like this."
A Belgian woman reported seeing a young girl who looked "very much" like Madeleine with an Eastern European couple on a tram in Brussels 12 days after she disappeared, the files reveal.
Line Compere, 31, told Belgian police she saw the blonde-haired, blue-eyed child on the number 18 tram heading towards the main Midi train station at about 8.45am on 15 May last year.
Her suspicions were aroused because the man and woman with the youngster did not look like her, appeared to be speaking a different language from her and did not seem to know her well.
At that point she had never seen a picture of Madeleine but later that day she was shown a photograph of the missing girl.
Ms Compere told police in her interview on 1 June last year: "When I saw the photo in question, it immediately jumped out to me that there was a big resemblance to the little girl on the tram."
The report was passed on to the Portuguese police but it is unclear what action was taken to follow it up.
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