Abu Qatada was allowed to film his plea as part of an unprecedented effort to secure the lives of Norman Kember, 74, and three other Western peace activists.

Officials admitted that it was a unique broadcast but, with the kidnappers threatening to kill their captives within the next few days, they were prepared to take the desperate measures.

Their hope is that the intervention of such a prominent Islamic militant may succeed in persuading the kidnappers from the Swords of Truth group after efforts by more than 20 Muslim groups have so far failed.

Last night the kidnappers extended the deadline for killing their hostages by 48 hours to Saturday night and released new footage of Mr Kember pleading for Britain to pull out troops from Iraq and for help to free him.

British diplomats are reported to have used more intermediaries than they did during the failed attempt to save Kenneth Bigley and Margaret Hassan. But few could have expected that ministers would turn to a man described by judges as “a truly dangerous individual”. Officials insist that they have offered no leniency to Abu Qatada in return for his role.

They claim that it was he who approached prison staff with an offer to intervene. Both the Home and Foreign secretaries gave permission after first checking with diplomats in Baghdad that Abu Qatada’s plea would not jeopardise undercover efforts to save Mr Kember from Pinner, northwest London, two Canadians and an American. The Prime Minister was also informed.

Abu Qatada was permitted to write his own address, but it was carefully vetted to ensure that there were no coded messages or any “inflammatory sentiments”. He was filmed at the top-security prison at Full Sutton, near York.

Dressed in a flowing white robe and looking notably thinner than at his arrest in August, Abu Qatada, speaking in Arabic, told the kidnappers: “I am your brother Abu Qatada, Omar bin Mahmud Abu Omar, who is imprisoned in Britain.

“I urge my brothers, the Brigades of the Swords of Right in Iraq, to release the hostages in line with the principle of mercy of our religion.

“Our prophet said mercy should be shown unless there is a reason in Sharia [Islamic law] that prevents it.”

He did not condemn kidnapping and was careful to emphasise that the appeal was on behalf of the four Christian peace activists only and did not include other Western hostages.

The appeal was filmed on Tuesday and his lawyers handed the tape yesterday to the Arabic satellite channels al-Jazeera and al-Arabiya.

Last night, new television footage on al-Jazeera showed two of the prisoners, Mr Kember and Tom Fox, 54, an American peace activist, dressed in orange outfits similar to those worn by prisoners at the US-run Guantanamo Bay detention centre. Previous Iraqi hostages have been pictured wearing orange jumpsuits, including Briton Ken Bigley who was beheaded by his captors last year.

In the tape the activists made statements condemning the US and British presence in Iraq.

“I ask Mr Blair, the British Government and the British People to work both for my release and for the release of the Iraqi people from oppression,” Mr Kember said.

Mr Kember, a retired medical physicist who was in Iraq working for the Christian Peacemaker Taskforce, was abducted in Baghdad on November 26 along with his colleagues, Mr Fox and Canadians James Loney, 41, and Harmeet Singh Sooden, 32.

Ministers hope that by allowing Abu Qatada’s broadcast they are demonstrating to militants in Iraq that they have enlisted all shades of Muslim opinion in Britain.

Mr Straw said last night: “Our overall position in respect of kidnapping has been made very clear already. These four men are all campaigners for peace, dedicated to the helping of others, and we ask for their release. If the kidnappers want to get in touch, we want to hear what they have to say.”

The Home Office said that yesterday’s plea had no bearing on Abu Qatada’s appeal against the decision to detain him pending his removal to Jordan. “The current legal processes will be decided on their merits,” a Home Office source said.

However, sources admitted that Abu Qatada was likely to use the fact that ministers had allowed him to make the appeal to bolster his case against deportation.

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