An Arabic TV channel has aired a video claiming to show one of five Britons captured in Iraq eight months ago.
The footage showed a man making an appeal to Prime Minister Gordon Brown to help secure the hostages' freedom.
"My name is Peter Moore, I have been held here for nearly eight months now," the man said in English.
He asks Mr Brown to free nine Iraqis in exchange for the release of the Britons seized in May 2007. The Foreign Office has condemned the release of the video.
The video shows the man speaking in English with an Arabic translation played over the top.
The man appealed for the freedom of the Iraqis prisoners - being held by the coalition authorities.
"All I want is to leave this place. I tell Gordon Brown the matter is simple: release their prisoners so we can go," he said.
The man, who is wearing a black and white tracksuit, says on the tape that he is from Lincoln and appeals to the British government to help secure the group's freedom.
"It's as simple as that. It's a simple exchange of people. That is all they want - their people and we can go home," the man said.
The Foreign and Commonwealth Office (FCO) criticised the footage and said: "We condemn the release of videos such as this, which are greatly distressing to the families of those involved. Our thoughts are with them at this difficult time.
"We urge those holding the group to release them immediately. We are in close contact with the Iraqi authorities and doing everything we can to try and secure a swift release."
The FCO said it wanted anyone who could influence the situation to do what they could to ensure the "safety and release of the hostages".
"We again call directly on those holding these men to release them," it said.
Security expert Sally Lievesley, a former Home Office adviser, said the man looked like he had been forced into giving the message.
She said: "We look at the message on the victim's face - the stress, the absolute fear and the utter need to do what he is being asked that tells us how involuntary that message is.
"Although he looks well kept physically, mentally, I think his family will be very upset at seeing that video."
BBC News security correspondent Frank Gardner said the kidnappers appeared to have mellowed their tone since the release of a previous tape in December 2007.
In that footage, one of the four British bodyguards seized along with IT consultant Mr Moore was flanked by gunmen and warned of an ultimatum.
Our correspondent said: "The good news is that in the extremely brutal and violent world of Iraqi abductions these men appear to be relatively moderate.
"They've got negotiable aims, although of course Britain doesn't officially negotiate with hostage-takers."
He added: "The fact that they are holding these men and appear to be keeping them alive, reasonably well fed, not chained up, and that they haven't apparently executed any of them after more than eight months is encouraging."
The five men were seized from Baghdad's Finance Ministry by gunmen wearing police uniforms on 29 May 2007 without a shot being fired.
The BBC's Jim Muir in Baghdad said little was known about the hostage-takers who call themselves the Islamic Shia Resistance in Iraq and they had not been heard of before.
He said they could be connected to the Mehdi Army militia or have possible links with Iran.
The video has not yet been authenticated but he said there was "little to doubt the veracity of it".