Abu Qatada is awaiting extradition to Jordan, where he faces life in jail
North African militants have demanded the release of a prisoner held in the UK, and threatened to kill a British hostage if their demand is not met.
In a website posting, al-Qaeda's North Africa wing said Abu Qatada, a radical preacher convicted of terrorism in Jordan, must be freed within 20 days.
They said they were holding a British man and a Swiss tourist, who were snatched in Niger on 22 January.
The UK Foreign Office called for the release of all captives.
"Hostage-taking can never be justified, no matter what the cause. We call on all those holding hostages to release them immediately and unconditionally," a spokesman said.
An Islamist website carried a statement from al-Qaeda on Sunday saying: "We demand that Britain release Sheikh Abu Qatada, who is unjustly [held], for the release of its British citizen.
"We give it 20 days as of the issuance of this statement. When this period expires the mujahideen will kill the British hostage."
Analysts say Islamist fighters are trying to spread their influence from their base in Algeria into countries such as Mali and Niger.
Abu Qatada is awaiting extradition to Jordan, where he was convicted of terrorism offences in his absence and faces life in jail.