John "Jackie" Elliott, who has been on death row for 16 years, is due to die by lethal injection in Huntsville tonight after being sentenced to death for the 1986 rape and murder of 18-year-old Joyce Munguia.

His lawyers have called on the Texas courts to approve DNA testing of key evidence from the original trial which they believe will prove his innocence. The call for a DNA test comes amid a flurry of last-minute appeals in Austin, Texas and lobbying from John Gummer MP, the Home Office and the British embassy in Washington.

Last week it emerged that Judge Jan Wisser, who was overseeing the case, had written to the Texas Board of Pardons and Parole, urging them not to grant clemency.

His letter said: "Never have I come into contact with a defendant more deserving of the ultimate penalty than Mr Elliott. I am totally sure he received a fair trial... the only tragedy is that this case has lingered so long." Mr Wisser had originally said that he would hear the case for his removal on Thursday, two days after Elliott is due to be executed.

After repeated requests by Elliott's defence team, led by the British lawyer Clive Stafford Smith, Mr Wisser took himself off the case and was replaced by Judge Chuck Campbell.

The defence team may also challenge Mr Campbell, who is known to have spoken with Mr Wisser last week, raising doubts about his impartiality.

Elliott, 42, who was born in Mr Gummer's constituency in Suffolk to American parents but left the UK when only a few months old, has protested his innocence since he was sentenced to death in 1987.

He claims members of the 'Chain Gang', a group of local criminals in east Austin, beat Ms Munguia to death with a motorcycle chain under a bridge after a night of drinking and drug-taking.

Elliott, who was not a gang member but was with Chain Gang members on the night, grew up on welfare in the mainly Hispanic Santa Rita housing projects in east Austin. When he was 18 he was sentenced to four years in jail for his involvement in a drive-by shooting with another gang.

Elliott's defence in this case hinges on disproving claims by the key prosecution witness, Daniel Hanson, who told the jury in the original trial that he witnessed the start of Elliott's murderous attack and ran for help. It is already known that Mr Hanson, who was a gang member, went to a bar, and then phoned his sister. She did not call the police for another 90 minutes.

The evidence the defence are seeking to have re-examined relates to the blood-spattered shoes Mr Hanson was wearing on the night of Ms Munguia's killing.

The trial took place at a time before DNA testing was common and police believed Mr Hanson's claim that the blood was his own, resulting from a fight two months earlier when he was stabbed in the back.

Recently disclosed documents reveal statements from two women who claim Mr Hanson had bragged to them of his role in the attack, saying it was he who hit Ms Munguia with the chain.

British efforts to have the execution either stayed or commuted have intensified in recent weeks.

Mr Gummer, who visited Texas two weeks ago to lobby on Elliott's behalf, said yesterday: "It is outrageous that they haven't taken DNA tests which are very necessary and which could prove his innocence.

"The latest evidence, uncovered in the last two or three days, is that there is a further witness who points the finger at the very person who accused him and who otherwise would have been in the dock, and on whose evidence the whole case stood."

"We have seen the original police documents which show that the police did not believe him to be guilty, but the person who in the end testified against him and thereby saved himself from execution.

"All I am hoping is that they will see that, even if you believe in capital punishment, the one thing it must mean is you have to be absolutely sure that you get the right person."

Despite the change of judge, Elliott's hopes remain slim. Texas, which carries out more executions than any other state in America, has rejected 289 appeals for clemency in a row, except for a single case where the condemned man was terminally ill.

It has never responded positively to a plea for clemency from a foreign government. "We have a good and compelling case," said Mr Stafford Smith. "The trouble is we're in Texas."

On Thursday the foreign secretary, Jack Straw, phoned the Texas Pardons and Paroles Board and asked it to commute Elliott's death sentence to a prison term.

The Bishop of London, the Rt Rev Richard Chartres, and the head of the Catholic Church in England, Cardinal Cormac Murphy-O'Connor, have also called for Elliott's life to be spared.

Last month the outgoing Republican governor of the state of Illinois commuted all execution sentences and released several death row inmates after persistent evidence that many were wrongly convicted.