In a videotape released on Islamist websites on Wednesday, Bigley pleaded with the Prime Minister to help him.

"This is possibly my last chance to speak to you," the presumed captive, whose voice breaks down several times in the footage, tells Blair.

"I need you [Blair] to be compassionate as you've always said you were ... I don't want to die... Please, please release the female prisoners that are held in Iraqi prisons," he says, breaking down in tears.

"I need you to help me now Mr Blair," Bigley said in the video, apparently made by the al-Tawhid and al-Jihad group of al-Qaida ally Abu Musab al-Zarqawi, which is holding him captive.

A spokesman for Blair said: "We are doing everything we can to try to resolve this situation. At the same time, everybody acknowledges how difficult this situation is."

Diplomatic instruction book

But in an interview with Aljazeera.net earlier in the week Bigley's brother, Paul, expressed frustration with the British government's action, saying they were going through the 'diplomatic instruction book'.

"I need you (Blair) to be compassionate as you've always said you were ... I don't want to die ... Please, please release the female prisoners that are held in Iraqi prisons" 
Kenneth Bigley,
British captive

"They [Blair and his government] are ignorant. They have no ears for the situation on the ground.

"The whole attack on Iraq was absolutely ridiculous, unfounded and stupid. And I will not be told to shut up, not about an important situation like this," he said.

Asked for his reaction to the video, Paul told Reuters: "It's good news is it not? The man is alive."

It was not clear if the move to release the video meant Bigley had been spared for at least another day.

Demands not met

Bigley and Americans Jack Hensley and Eugene Armstrong, whose UAE based construction firm operates at US bases in Iraq, were seized last week and threatened with death unless women prisoners in Iraqi jails were released.

Their captors beheaded Armstrong on Monday and said on Tuesday they had killed Hensley, whose body was later found.

In Wednesday's appeal, Bigley, who sits with a flag of Zarqawi's group behind him, sobs as he speaks about his desire to see his family.

He is dressed in orange overalls typical of US jails and associated with images of detainees at Guantanamo Bay.

"I think this is my last chance to speak. I don't want to die in Iraq, neither do the women in the prisons," Bigley said.