The witness, who saw a man at the murder scene, apparently rules out Eddie Browning - sentenced to life imprisonment in 1989 - as the killer. Interviewed by West Mercia police at the time of the killing, the witness's identification evidence was not introduced at Browning's trial.
It will be revealed tonight in BBC Wales's Week In Week Out programme, which has spent several months inquiring into the case.
The Police Complaints Authority is already investigating allegations that West Mercia detectives halted vital lines of inquiry, improperly documented aspects of the case, and did not disclose evidence. A spokesman said last night that it would look at any new matters raised.
Browning's lawyers, who want the case sent back to the Court of Appeal, have already gathered a dossier of new evidence, which they say casts serious doubt on the conviction. Jim Nichol, Browning's solicitor, said yesterday that the new evidence was significant and that there should be no further delay in re-examining the conviction.
Marie Wilkes was 22, and seven months pregnant, when she was killed after her car broke down on the M50 near Bushey, Hereford and Worcester.
Leaving her two young children in the car, she walked along the hard shoulder to call for help from an emergency telephone. She was accosted by a motorist, who cut her throat and drove her a few miles up the motorway before throwing her down an embankment to bleed to death.
Browning, 39, a night-club bouncer from Cwm-parc, Mid Glamorgan, was convicted after the jury heard that he had been in a furious row with his wife, also pregnant, on that day and that a car similar to his silver Renault 25 was seen near the murder scene. But there was no forensic evidence linking him to the murder.
In tonight's programme, John Hayward, a former Home Office forensic scientist, concludes it was 'unlikely' that Mrs Wilkes was ever in Browning's car.
He concludes that the stab wound would have caused her to bleed 'significantly on to the seats and carpet of any motor car into which she was transferred'. Even if cleaned, he thought blood traces would remain. 'Fabric car seats, even if thoroughly sponged, I would expect them to still give only a weak, but a chemical reaction, indicating the presence of blood,' he says.
In May, Greater Manchester Police were asked to investigate why a videotape of a West Mercia policeman about to undergo hypnosis to assist with his recall was not disclosed to the Crown Prosecution Service or the defence at the time of his trial.
The film shows Inspector Peter Clarke, who saw the murderer's car while driving on the motorway, recalling a number plate different to the one on Browning's car.Reuse content