The brother of British hostage Kenneth Bigley says the US has "sabotaged" his brother's release by refusing to free a detained woman scientist in Iraq.
A grainy 11-minute video carried the plea by the 62-year old hostage
Paul Bigley told the BBC there had been "a shadow of light" when Iraqi ministers said the woman would go free.
But the US ruled out freeing the woman - one of two held in Iraq - saying it would not give in to the kidnappers.
Kenneth Bigley was seen in a video appealing to UK Prime Minister Tony Blair to help save his life.
His Thai wife made an appeal afterwards asking the captors for mercy.
"My husband, Ken, is an ordinary, hardworking family man who wanted to help the people of Iraq among whom he made many friends. As a loving wife, I beg you once more for mercy," Sombat Bigley said.
The Iraqi government insisted on Thursday that it would not release two female weapons scientists despite statements on Wednesday that it would.
Britain has ruled out negotiations with the kidnappers.
The Tawhid and Jihad Group headed by al-Qaeda suspect Abu Musab al-Zarqawi have threatened to kill Mr Bigley unless the US and the UK release all women held in Iraqi jails.
However, they have never referred specifically to the two women concerned and, correspondents say, may never have heard of them.
The kidnappers have already killed two American hostages.
The US says it is holding two female weapons scientists, Rihab Rashid Taha and Huda Salih Mahdi Ammash. The UK says it has no Iraqi women in its custody.
Meanwhile the Italian government has dismissed as "unreliable" claims posted on the internet that two Italian women held hostage in Iraq had been killed.
Nothing to negotiate
Paul Bigley told BBC Radio 4's Today programme there had been "a shadow of light in a big, long, dark, damp, filthy, cold tunnel" when it appeared Dr Taha would be freed.
"Now this has been sabotaged," he said.
"The fact on the table now is that nobody has to negotiate with
anybody about anything," he said.
"All the powers have to do now is allow the Iraqis to conduct their own internal affairs the way they should be doing."
UK Foreign Secretary Jack Straw said the hostage had been placed in an appalling situation by evil men, but this could not alter the UK's position.
The 11-minute video of Mr Bigley's plea was apparently made by the group.
In it the hostage appeals to Mr Blair, saying: "You are the only person on God's earth who can help me."
"I think this is possibly my last chance," he says, sobbing repeatedly.
"I don't want to die... Please free female prisoners held in Iraqi prisons."
Mr Bigley's relatives have urged the kidnappers "to be all merciful".
RIHAB RASHID TAHA
Nicknamed Dr Germ
Worked on Saddam Hussein's biological weapons programme
Accused of producing anthrax
Educated in UK's University of East Anglia
Surrendered in May 2003
Considered important target, but not on US most-wanted list
Iraqi Minister of State Kassem Daoud denied that his government had bowed to
US pressure in deciding not to release Dr Taha.
The decision was made by interim Prime Minister Iyad Allawi in order to show
that Iraq would not negotiate with the kidnappers, he told Today.
"Unfortunately... there was an impression that the Iraqi government made a deal with the terrorists, which we never, ever do," he said.
Mr Bigley, 62, was among three Western civilian contractors kidnapped a week ago in Baghdad.
Video footage released on Monday showed American Eugene Armstrong being killed by a masked man said by the CIA to be Mr Zarqawi.
A second American, Jack Hensley, was killed 24 hours later. A video purportedly showing his killing was posted on Wednesday evening after his body had been found.