A video posted on an Islamist website shows a British hostage in Iraq pleading for his life.
Kenneth Bigley spoke for several minutes
Militants have threatened to kill Kenneth Bigley unless the US and UK release all Iraqi women held in the country's prisons.
The grainy 11-minute video appears to show Mr Bigley pleading to UK Prime Minister Tony Blair to save him.
Two US contractors kidnapped with Mr Bigley last week in Baghdad have already been killed.
Speaking after his father appealed to Mr Blair, Craig Bigley, 33, said in a statement on behalf of the family: "Be merciful, as we know you can be. Release Ken back to his wife and family.
"We ask you as a family to be all merciful."
The US is holding two female weapons scientists, Rihab Rashid Taha and Huda Salih Mahdi Ammash, but says it has no plans to release them.
The UK says it has no Iraqi women in its custody.
Foreign Secretary Jack Straw said the British government was doing everything it could to secure Mr Bigley's release but could not give in to terrorists.
He added that it would be idle to pretend that there was a great deal of hope.
Reacting to the video, he told the BBC: "It's adding torture to the appalling situation which these evil terrorists have placed Mr Bigley in. Of course our hearts go out yet more to him and his family.
"[But] I'm afraid to say it cannot alter the position of the British Government."
Meanwhile, an Iraqi militant group that kidnapped two Italian women in Baghdad is reported as saying it has killed them.
In a message posted on the internet, a group calling itself the Jihad Organisation said the women were killed because Italy had not heeded a call to withdraw its forces from Iraq.
There has been no confirmation of the report, either from within Iraq or from the ltalian authorities.
In the video, Mr Bigley says: "I need you to help me now Mr Blair, because 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. I don't deserve [it]. Please free female prisoners held in Iraqi prisons."
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 to US forces in May 2003
Considered important target, but not on US list of 55 most wanted Iraqis
"Please, please help me see my wife, who cannot go on without me," he added.
The hostage is dressed in an orange jumpsuit similar those worn by prisoners in US detention facilities, including Muslim inmates at
The video was apparently made by the Tawhid and Jihad Group headed by al-Qaeda suspect Abu Musab al-Zarqawi.
A Downing Street spokesman described the situation as "very difficult".
Mr Bigley's brother Paul describing Ken as a "man of great tenacity".
But he criticised the US for what he described as "throwing egg in the face" of Iraqi officials, after Washington apparently contradicted them for saying the women might be released soon.
The three hostages, who worked for Middle East-based general services and construction contractor Gulf Supplies and Commercial Services, were kidnapped on Thursday in Baghdad.
Gruesome video footage was released on Monday showing Eugene Armstrong being killed by a masked man thought to be al-Zarqawi.
Jack Hensley was killed 24 hours later, and a video purportedly showing his killing was posted on Wednesday evening after his body had been found and identified.
Mr Bigley, a 62-year-old from the city of Liverpool, is married with one child.
Two Iraqi ministers said earlier that on Wednesday that Dr Taha and Dr Ammash might be released in the near future.
But Foreign Minister Hoshyar Zebari told the BBC's Newsnight programme that Rihab Taha was a "highly valued prisoner" and there were "no immediate plans" to let her go.
He added that such decisions were not taken by individual ministers but by the whole government.
"These two ladies have poisoned 1,000s of Iraqis and they should face justice," he said.
Dr Taha is said to have carried out top-secret work during the 1980s on germs that cause botulism poisoning and anthrax infections.
Dr Ammash, a biotech researcher known as Mrs Anthrax or Chemical Sally, was on the US military's list of the 55 most-wanted members of Saddam Hussein's regime.