One of Britain's highest-profile Islamic extremists issued a televised appeal yesterday for the release of hostage Norman Kember in Iraq.

In an extraordinary development, Abu Qatada, who has been described by intelligence agencies as Osama bin Laden's ambassador in Europe, was recorded making the appeal inside Full Sutton maximum security jail, near York, where he is awaiting possible deportation from the UK.

Last night, it was reported that the deadline for the execution of Mr Kember and three other Christian peace activists had been extended by two days, for the US and British governments to meet the kidnappers' demands.

Meanwhile, British officials said Mr Qatada's request to make an appeal came from him through his lawyer, Gareth Peirce.

The Prison Service gave permission and the filming was done internally rather than by an outside media organisation and in the past 24 hours. The tape was then distributed to satellite news stations in the Middle East.

Speaking in Arabic, Mr Qatada appeals to the Swords of Righteousness Brigade, which had reportedly threatened to kill the hostages if Iraqi prisoners were not released from US and Iraqi jails by today. That deadline has now been moved to Saturday.

He says he is their "brother" but adds: "I urge them to release the four prisoners in Iraq. This is a merciful act according to the principles of Islam."

The Special Immigrations Appeals Commission has in the past described Mr Qatada as "a truly dangerous individual" and a key figure in the UK in al Qaida-related terrorist activities.

In evidence to the commission in 2003, the then Home Secretary, David Blunkett, said Mr Qatada was the most significant extreme Islamist preacher in the UK and he had been "an inspiration" for terrorists, including Mohammed Atta, the lead hijacker behind the September 11 attacks.

He is also said by the British Government to have "extensive contacts with senior terrorists worldwide".

Last night, Foreign Secretary Jack Straw renewed calls for Mr Kember's release.

Referring to the extended deadline, he said: ''The message of this latest statement is not clear. If the kidnappers want to get in touch, we want to hear what they have to say.''

Mr Kember, 74, from Pinner, north-west London, was seized in Baghdad on November 26 with James Loney, 41, and Harmeet Singh Sooden, 32, both Canadians, and an American, Tom Fox, 54.

He had travelled to Iraq as a gesture of solidarity with the Christian Peacemaker Team, a Canada-based international peace group.

Video footage has appeared on the Internet showing Mr Kember in handcuffs and shackles and saying: "I ask Mr Blair to take British troops out of Iraq and leave the Iraqi people to come to their own decisions on their government."