A series of discrepancies in the evidence against the women, and doubts over the legal advice they obtained in Thailand, have prompted Graham Allen, Labour's home affairs spokesman, to ask both the Foreign and Home Office to look again at the case.
Karyn Smith, 21, and Patricia Cahill, 19, were arrested at Bangkok airport in July 1990, allegedly attempting to smuggle heroin worth pounds 4m.
Cahill has subsequently admitted that she knew she was 'carrying a little something' but thought it was gems not drugs. Smith, however, has always maintained she was an innocent dupe - along for the ride and not knowing of her friend's plans and arrangements. Despite this, she was apparently advised by her Thai lawyers to plead 'guilty but ignorant' because she would be treated more leniently by the courts. Thailand carries the death penalty for drug trafficking, although it has never executed a foreign national.
In the event, Smith was sentenced to 25 years and Cahill, at the time a juvenile and who had pleaded not guilty, was convicted and sentenced to 18 years. According to the Smith family and their lawyer, Stephen Jakobi, there were sufficient doubts in her case to enter a not guilty plea.
Further inconsistencies and anomalies in the evidence at both the women's trials went unchallenged.
Acting on a tip-off Thai police had the pair under surveillance. But when a man arrived at their hotel to hand over the drugs, he was apparently neither followed nor arrested.
'Surely he would have been a far more lucrative catch for the Thai authorities than two girls, who were at most mules?' Mr Jakobi said.
Even more curious, he says, is the amount of heroin the women were said to have been carrying. About 30kg (66lb) was said to have been concealed in compartments in 12 shampoo bottles, seven drinks cans and nine sweet tins.
Smith alone was said to have seven cans and seven bottles, found to contain, according to the Thai judge '10 bags of white powder that weighed approximately 13.5 kilos'.
Mr Jakobi said: 'It is quite simply physically impossible for that amount of heroin to have been concealed in those bottles. Whatever was originally found in the women's cases was not what was presented in court. That alone should be enough to raise serious doubts about the entire prosecution.'
Experience shows that drug couriers - mules as they are known - carry at most about 1.5kg, and Mr Jakobi maintains the women were planted with most, if not all, of the drugs. He said that might account for the fact that they were arrested before checking in for their flight. Their baggage would have been weighed and the weight recorded, which would not have allowed for any extra amounts to be added, he claims.
Both women have spent two and a half years in jail and their hopes of freedom lie in pleas submitted nearly a year ago for a pardon from the King of Thailand.
Mr Jakobi is critical of the way the case had been handled by both the Foreign and Home Offices and his dossier on the case has prompted Mr Allen to table a series of parliamentary questions today.
The MP said: 'I think there are several aspects of this case which cause grave concern and I am asking the Foreign Office and the Home Office to look again at the details of the matter in case any injustice has taken place.'
Yesterday Eric Smith, who has just returned from visiting his daughter in Thailand, said she was 'bearing up very well under the circumstances'.
'I am glad questions are being asked about the case for both of the girls' sakes,' he said.