Peter Bleksley believes the shirt worn by the prime suspect was a rare design produced by beer company Super Bock.
It was not sold but given away to loyal customers and should be possible to trace.
The suspect is a child sex attacker who has been operating for years in the area of Portugal where Madeleine vanished in 2007.
His burgundy coloured top with a distinctive white circle on the back was described by the families of two of his victims.
The Yard has released a picture of the long-sleeve T-shirt.
Mr Bleksley, 54, a founder member of the Yard’s undercover squad, said: “Some people have suggested it bore a resemblance to a strip once worn by the Arsenal football team.
“However, I think a more likely explanation could be that the man was wearing one of these Super Bock promotional T-shirts.
"From speaking to bar owners I know the design for these T-shirts changes every year and only a certain number are handed out to regular customers.
“Therefore, it would be possible to check back on all the designs and the years they were produced and see which one bears most similarity to that witnessed by families of victims.
“Then it would be possible to check the distribution of the T-shirts to see where they were given out.
"Not all pubs bother with the promotion so through a process of elimination it may be possible to narrow down where such shirts may have been handed out and then bar owners could be asked about their customers.
“We know that some victims said the man smelt of stale alcohol and tobacco and that he had a pot belly, so that would suggest he was a regular drinker or may have worked in a pub or restaurant.
"We know that some victims said the man smelt of stale alcohol and tobacco and that he had a pot belly, so that would suggest he was a regular drinker or may have worked in a pub or restaurant
“The man appears to target his victims after watching families and working out how to get into their holiday apartments, suggesting he is methodical and has his own modus operandi.”
The distinctive Super Bock logo of a white circle on a maroon background is used on signs outside pubs across Portugal.
In the Algarve resort of Carvoeiro we found a tradesman wearing one of the shirts with the words Super Man on it. He said he had owned the shirt for a number of years.
Last week the Sunday Express flew Mr Bleksley to the Algarve to assess the progress of the Yard’s investigation into Madeleine’s disappearance from her family’s holiday apartment in Praia da Luz on May 3, 2007, seven years ago this Saturday, when she was approaching her fourth birthday.
He arrived just as Scotland Yard announced that there were five more attacks on British girls in and around Luz, including a disturbing sexual assault on a girl of 10 in 2005 in the very resort from where Madeleine disappeared two years later.
The Yard are looking at 18 potentially linked cases of someone entering holiday homes.
They are focusing on 12 break-ins during which nine girls were attacked and three other cases described as near-misses.
Mr Bleksley said: “The Yard has a massive job on its hands to look into all these crimes and every tiny clue is important.
"The one solid clue comes from two families who report the intruder as wearing the top with the distinctive white circle.”
Mr Bleksley, whose book on unsolved murders, called Ten Most Wanted, highlighted shortcomings in police investigations, has analysed the McCann files for the Sunday Express.
He suggested it was time to go back to square one, with a new forensic sweep of material from the holiday apartment where she vanished.
Last week Detective Chief Inspector Andy Redwood revealed that some of the attacks on British girls had been reported to the GNR, the military-style police force that investigates minor crimes, but it appears the information was not passed on to the Policia Judiciaria, equivalent of the British CID.
Mr Bleksley said: “I was very alarmed to hear Mr Redwood suggest there may be forensic material relating to some of these attacks sitting in a GNR cupboard somewhere.
"Clearly, if this is the case these exhibits need to be found urgently.
“In fact, I would go further and say there needs to be a full re-examination of all known material on all 18 of the break-ins and particularly those during which girls were assaulted.”
He believes there should also be a full review of all forensics relating to Madeleine’s disappearance from apartment 5a of the Ocean Club.
MADELEINE MCCANN: Sunday Express special investigation seven years on from disappearance
“There was a case I looked into for my book where a review of all the forensics on the murder of a woman was re-examined 10 years after the crime and a breakthrough was made which led to a charge of murder.
“Forensic techniques are continually evolving and improving and I think the Yard should consider whether there would be any value in conducting further forensic tests of material from apartment 5a.
“An abductor may have come into contact with bed linen, door handles, door frames, walls, furniture and there could be minute traces from which it was not possible to obtain a profile then but may be possible now.
“If a profile was obtained from 5a it would be very interesting to see if it matched profiles taken from scenes of crime where British girls were attacked in apartments or villas.
“I sincerely hope the Met and the PJ work in tandem towards a shared goal. If these offences had happened in Britain, the victims would have been interviewed on video and the information processed and cross referenced to see if there were similarities to other reported offences and to see if a pattern emerged.
“This obviously was not done in Portugal so the Yard had to go right back to square one.”
http://www.express.co.uk/news/uk/472668/FOUND-Shirt-worn-by-Madeleine-McCann-suspect
