The man featured in two e-fits released by UK police investigating Madeleine McCann's disappearance was seen carrying a child towards the beach.
Police say a family saw the man with a blond-haired child of three or four, possibly wearing pyjamas, heading away from the McCanns' holiday apartment.
Madeleine, from Rothley, Leics, was three when she went missing on 3 May, 2007, in Praia da Luz, Portugal.
The e-fit images will feature in a BBC Crimewatch appeal later.
The witnesses said the man was white, 20 to 40 years old and of medium build. He had short brown hair, was clean-shaven and of medium height, they added.
Det Ch Insp Andy Redwood, the senior Metropolitan Police investigating officer, said he could be the man who took Madeleine but there could be an innocent explanation.
The Met Police opened up a new investigation earlier this year and have been reassessing all the gathered material.
As a result, according to the police, the timeline and "accepted version of events" surrounding Madeleine's disappearance have significantly changed.
Det Ch Insp Redwood said he was now attaching more weight to the sighting of the man and the child at 10pm, which was later than the time it was previously believed Madeleine was taken.
Madeleine and her brother and sister were left in the apartment at 8.30pm while her parents dined with friends at a nearby restaurant. Her father, Gerry McCann, checked on them at 9.05pm and her mother, Kate, raised the alarm at 10pm.
Det Ch Insp Redwood said the child in the man's arms had not been crying, nor looked in distress.
He added: "Whilst this man may or may not be the key to unlocking this investigation, tracing and speaking to him is of vital importance to us.
"This is far from our only line of inquiry and there will be e-fits released of other sightings as well, who we are equally keen to trace.
"These people were seen on the day of Madeleine's disappearance and the days leading up to it."
Thursday 3 May 2007: Timeline
- 20:30 Kate and Gerry McCann leave their apartment to have dinner at a Tapas bar
- 21:05 Gerry McCann checks on Madeleine and her siblings
- 22:00 A man is seen carrying a child wearing pyjamas heading towards the ocean
- 22:00 Kate McCann raises the alarm that Madeleine has gone missing
Prime Minister David Cameron said he was "very pleased" Scotland Yard were doing this work, which the government was helping to fund.
"This was a crime that touched the heart of everyone in the country and everyone would like to see it resolved, so I hope Scotland Yard continue with their work and I wish them success," he said.
A detailed reconstruction lasting close to 25 minutes and covering events leading up to and surrounding Madeleine's disappearance will also feature on Monday's Crimewatch.
A short clip released in advance by police shows actors playing Madeleine's parents leaving their holiday apartment for the nearby tapas restaurant.
They are then seen seated outdoors at a large poolside table, greeting friends as they come to join them for dinner.
In 2008, a Portuguese investigation into the case was dropped, but Scotland Yard began a review of the case in May 2011 and opened a formal investigation in July this year.
Speaking about the review-turned-investigation, Det Ch Insp Redwood said detectives had been able to "make massive steps forward" by drawing together all the material gathered to date and reviewing it as a whole.
He said his team had sought to "try and draw everything back to zero... take everything back to the beginning and then reanalyse and reassess everything, accepting nothing".
He added: "Praia da Luz is a popular holiday destination for many nationalities so our requests for help need to be repeated in many different countries."
He said he would travel to the Netherlands, Germany and Ireland to seek public support there, and would repeat the appeals in Portugal - "a key country for us to trace any outstanding witnesses".
In Germany, a special edition of the crime programme Aktenzeichen XY - Ungeloest which is translated as "File XY - Unsolved", will be aired on Wednesday night and will feature an appeal for information from Mr and Mrs McCann.
'Not to blame'
Former Child Exploitation and Online Protection Centre head Jim Gamble - who carried out a review into the case in 2010 - said the appeal offered "an opportunity for everyone to help".
"It doesn't matter that you were not in Praia da Luz - you may watch something, you may see an e-fit, you may see some other information, they may talk about telephone numbers and something will click with you," he told BBC News.
"And that you will not know unless you watch."
Mark Williams-Thomas, a former detective on the case, said it was "slightly disappointing" that the appeal was not being made in Portugal.
Mr and Mrs McCann are expected to make a live appeal from the studio during the programme.
Ahead of its broadcast, Mrs McCann said: "We're not the ones that have done something wrong here.
"It's the person who's gone into that apartment and taken a little girl away from her family."
Meanwhile Robert Murat, the first official suspect in the case but long since officially cleared of any involvement, told the BBC he hoped the new appeal would produce results.
"It's gone on and on and every time something new comes up, you know, it brings hope and then it seems to fade away and disappear. So I'm hoping that's not going to be the case again," he said.
The Crimewatch appeal will be shown in the UK on BBC One on Monday from 21:00 BST. It will also be broadcast in the Netherlands and Germany.