Abu Qatada, a radical Islamist regarded as al-Qaeda's ambassador in Europe, today made an extraordinary televised appeal for the safe release of Norman Kember, the British man being held hostage in Iraq.

Kidnappers who captured Mr Kember, 74, and three other peace activists in Baghdad last month, have demanded the release all Iraqi prisoners to secure his freedom. The deadline expires tomorrow.

Jack Straw, the Foreign Secretary, today said the demand - made by a group calling itself the Swords of Righteousness Brigade - was impossible, and described Mr Kember's situation as desperate.

In an unprecedented intervention, the Dubai-based Al-Arabiya television station today broadcast a short video message from Abu Qatada, who has been held in HMP Belmarsh since August 11.

Appealing directly to the kidnappers in Arabic, Abu Qatada said: "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 them [the four hostages] in line with the principle of mercy of our religion, if there was no compelling religious duty against it."

British officials said the request to make an appeal came from Abu Qatada himself through his lawyer, Gareth Peirce.

The Prison Service gave permission and the message was filmed yesterday. The tape was then released for broadcast to satellite news stations in the Middle East, including al-Jazeera.

A Jordanian state security court issued two 15-year jail sentences against Abu Qatada in absentia in 1998 and 2000 on charges of financing a banned Islamist extremist group, links with al-Qaeda and plotting to attack Christian visitors to the holy lands. Abu Qatada is being detained in Britain under anti-terror laws as he fights his extradition.

In Brussels, where he was attending a meeting over the European Union budget, Mr Straw said he was not in contact with the kidnappers of Mr Kember, from Pinner, northwest London, but added: "We are aware of these so-called demands which no government could meet. I am afraid I have nothing further to report. We work and we pray for a satisfactory outcome."

Meanwhile, Mr Kember's family released a statement through the Foreign Office in which they described him as a friend to the Iraqi people.

The family's statement read: "This is an extremely worrying, stressful time for all of Norman’s family. We are praying with people from all faiths for the safe release of Norman and his friends.

"My husband Norman doesn’t believe in violence and neither does his family. We believe as he does that everyone should live in peace. That is why Norman went to Iraq, because he wanted the Iraqi people to know that there are many people who are sorry for all their suffering. We are missing him, especially his three year old grandson.

"We especially welcome and deeply appreciate the support of the 25 Muslim authorities around the world, who have called on the people holding Norman and his three friends to release these innocent hostages immediately."

Mr Kember is a lifelong peace campaigner and human rights advocate. He was kidnapped with two Canadians and an American working for a Christian peace group on November 26.

Times Recommends

• Michael Jackson: Not the final chapter
• Indian Army spends10m rupees on golf buggies
• Sacha Baron Cohen forced to tone down Brüno