Detectives investigating the disappearance of Madeleine McCann are looking for a lone intruder who sexually abused five girls during break-ins at holiday homes.
The tanned, dark-haired man is suspected of breaking in to 12 properties where British families were staying in the Algarve, Portugal, between 2004 and 2010.
In four of the incidents, a total of five girls aged between seven and 10 were sexually assaulted. These attacks happened between 2004 and 2006, before Madeleine vanished in 2007.
Detective Chief Inspector Andy Redwood said tracing the man, who has "a vile interest in young, white, female children", is "a primary concern".
Madeleine's parents, Kate and Gerry McCann, were fully briefed on the latest developments in the investigation before they were made public today.
Mr Redwood said: "I'm sure the public will understand that the significant features of this offending - a man with a vile interest in young, white, female children, who he is attacking in their beds while they are on holiday with their families - has got a very close resonance to some of the features of Madeleine's disappearance.
"We really need to identify the offender, to bring to a close the trauma and the tragedy that these families have suffered, and then seek to establish whether this is connected to Madeleine's disappearance."
The potentially linked break-ins had previously been discounted by Portuguese investigators because they were spread over a wide geographical area and there were no apparent attempts at abduction.
Four of the burglaries were in Carvoeiro, six in the Vale da Parra, Praia da Gale district, and two in Praia da Luz.
Nine of the 12 incidents were reported to Portuguese police at the time they happened, but British investigators only became aware of three of those when the victims came forward in response to televised appeals last autumn.
In six of the offences, the man sat on or got into bed with young girls and, even when he was disturbed, witnesses said he remained calm.
He spoke English slowly, with a foreign accent and possibly slurred speech, had unkempt hair and was unshaven, and smelled strange. Some witnesses said he had a pot belly.
In two of the break-ins he was wearing a distinctive burgundy long-sleeved top, which some witnesses said had a white circle on the back.
Mr Redwood said: "This is an offender who has got a very, very unhealthy interest in young, white, female children who he is attacking whilst they are on holiday in their beds.
"While I completely accept that there are differences (between the break-ins and the McCann case), there is no abduction that we can see, but the assumption from that is that Madeleine McCann has been abducted. That may not necessarily follow with all our thinking about what may have become of Madeleine McCann.
"It is really critical for us to identify this offender and prove or disprove whether he was involved in Madeleine's disappearance."
Mr Redwood said that if names are put forward for the most serious assault, his team will be able to eliminate suspects from their inquiry. This suggests that police have DNA on file.
The investigators have 38 people classed as "persons of interest" in the investigation and are also sifting through details of 530 known sex offenders whose whereabouts they cannot account for. Of those, 59 are classed as high priority and some are British.
Last autumn the Scotland Yard team appealed for help in identifying a man who was seen carrying a child towards the sea on the night that Madeleine, then aged three, vanished, as well as a group of men who were seen lurking near the holiday apartment where her family was staying.
So far they have not been able to eliminate any of the men from their inquiry.
The British investigators have sent three international letters of request to Portuguese authorities for help with their inquiry, covering 41 priority areas for the team, involving 287 separate requests.
Deputy Assistant Commissioner Martin Hewitt said he is frustrated at how slow the legal process has been. "That's causing us frustration because we know what we want to do and we are ready to go with that. But the process is the process," he said.
Another 30 letters have been written to other European countries, but the force would not reveal where. However officers have travelled to Spain, Belgium, Jersey, Switzerland, Holland and Germany as part of the probe.
Madeleine, who was then nearly four, disappeared from her family's holiday apartment in Praia da Luz in the Algarve on May 3 2007 as her parents dined at a nearby restaurant with friends.
British detectives launched a fresh investigation into the youngster's disappearance in July last year - two years into a review of the case - and made renewed appeals on television in the UK, the Netherlands and Germany.
After shelving their inquiry into Madeleine's disappearance in 2008, Portuguese authorities said last October that a review had uncovered enough new information to justify reopening it.
Mr Redwood appealed for anyone else who may have been a victim of the intruder to come forward.
He said: "We need to establish the identity of this man. These offences are very serious and no-one has been charged in connection with them. We also need to eliminate this man from our inquiries and ascertain whether these offences are linked to Madeleine's disappearance.
"If you have been a victim of a similar crime, please come forward, even if you reported the incident to police in Portugal, or anywhere else. Please do not assume we have been made aware of it."