Civil rights group Liberty says it has concerns about the trial in Thailand of the two fishermen accused of murdering British student Katherine Horton.
The suspects appeared in court at Surat Thani on Friday
Dennis Eady, from Liberty, said there had been little "realistic defence" for the men in the "extremely quick" trial.
But he said there appeared to be a strong evidence against Wichai Somkhaoyai, 24 and Bualoi Posit, 23.
The body of Miss Horton, 21, from Cardiff, was found in the sea off the island of Koh Samui on 2 January.
Posit and Somkhaoyai admitted rape and conspiracy to kill at the court in Surat Thani on Friday.
The case has been adjourned until next Wednesday when the men face sentencing - just over two weeks after the death of Miss Horton.
Mr Eady, from the south Wales branch of the campaign group, told BBC Radio Wales: "It has been pushed through extremely quickly and in a context of massive political and commercial pressure.
"I suppose what would concern us is the lack of due process that seems to be taking place here."
He said that from a Western perspective he would have liked to have seen the two men have a "clearer defence".
"It does seem that these people don't seem to have had a realistic defence at all," he said.
Katherine Horton's body was found on 2 January
Thailand's prime minister Thaksin Shinawatra has declared that the men should face the maximum penalty because of damage done to the country's image.
Mr Eady added that Liberty was worried about the "plea-bargaining situation" around the death penalty in Thailand.
"People who plead guilty are less likely to get the death penalty," he said.
"That does inevitably create a perverse incentive to plead guilty and someone who is poorly defended might well be inclined to plead guilty simply to save their life, even though they could conceivably be innocent," he said.
But Mr Eady said the case was not necessarily a miscarriage of justice.
He added: "The evidence does seem to be strong here... Clearly this is a terrible crime and we have great sympathy with the family."