Detectives searching for Madeleine McCann hope that information released today will jog someone's memory and unlock a crucial clue to finding her.
Police have issued two images of a man they urgently want to trace as part of the investigation.
The pictures, showing a white male, aged between 20 and 40, with short brown hair, were compiled by separate witnesses who both spotted him in the resort of Praia da Luz on the evening the 3-year-old vanished.
While the two pictures appear to show a person with slightly different features, detectives are convinced it is the same person they are seeking.
The images will be broadcast this morning (NZ time) on the BBC's Crimewatch programme as part of a renewed international appeal.
Detectives from Scotland Yard, who have been leading a fresh investigation into Madeleine's abduction, will use the programme to reveal important new thinking around the timeline of the events leading up to her disappearance.
A detailed reconstruction of the McCann family's movements on the afternoon and evening of May 3, 2007, will be broadcast in the hope that it will trigger someone's memory..
More than 440 witnesses have been spoken to by officers, who have spent two years reviewing evidence collected by Portuguese police and private detectives.
As well as identifying 41 "persons of interest", the Scotland Yard inquiry - dubbed Operation Grange - has thrown up a number of potential suspects who witnesses have been able to describe in detail.
A spokesman for the Operation Grange team said detectives now had a far more detailed understanding of the times when an abduction could have taken place and this had given the statements from two witnesses particular significance.
Detective Chief Inspector Andy Redwood, the senior investigating officer, said: "Today I am asking the public for their help.
"While this man may or may not be the key to unlocking this investigation, tracing and speaking to him is of vital importance to us.
"We have witnesses placing him in the resort area around the time of Madeleine's disappearance.
"This is far from our only line of inquiry and there will be [pictures] 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."
The images are seen as an important step forward in the investigation, but it is hoped the Crimewatch reconstruction will throw up some fresh lines of inquiry.
The appeals will also be broadcast in Germany and Holland where many Praia da Luz tourists were from.
This morning's broadcast will also feature an emotional appeal from Kate and Gerry McCann, who describe their continuing torment.
Gerry McCann will say: "When it's a special occasion, when you should be at your happiest and Madeleine's not there, that's when it really hits home."
* Madeleine, 3, disappeared on the evening of May 3, 2007 as she slept in her family's holiday apartment in the Algarve with her younger brother and sister.
* Her parents were dining a short distance away with a group of friends at a tapas restaurant within the resort's complex and realised she was missing when they checked on her at around 10pm.
* The Portuguese authorities shelved their investigation into Madeleine's disappearance in July 2008, forcing her parents, Kate and Gerry McCann, to employ teams of private detectives to continue the hunt.
* But in May 2011, the British Home Secretary ordered Scotland Yard to begin a review of all the evidence.
* This year the Metropolitan Police announced the review was being turned into a formal investigation with more than 30 detectives and police staff working on the case full time.
* The Policia Judiciaria agreed to co-operate and there are now six dedicated officers based in Faro in the Algarve.