By Frank Gardner
BBC security correspondent
President Obama has vowed to close the controversial prison
A former Afghan inmate at the US prison in Guantanamo Bay has joined the Taleban's high command in Pakistan, UK government officials say.
They say Mullah Abdul Kayum Zakir, who was released last year, is now closely involved in planning attacks on British and other Nato forces in Afghanistan.
They say he is operating with impunity from the Pakistani city of Quetta.
The Pentagon says more than 10% of 520 inmates released so far have returned to what it calls terrorism.
It says this complicates efforts to release and repatriate those still being held.
US President Barack Obama has long vowed to close the controversial Guantanamo Bay detention centre.
Lord Carlisle's warning
British government officials say Mullah Abdul Kayum Zakir was handed over by the Americans in the spring of 2008 to the Afghan authorities, who briefly imprisoned him in Kabul.
But from there he was released and promptly made his way over the Pakistani border to rejoin the Taleban and its leadership.
Since then, says a US counter-terrorism official, he has had a hand in doing some very bad things.
Meanwhile, Lord Carlisle, the British government's independent reviewer of anti-terrorism laws, warned on Monday against imposing control orders on former Guantanamo inmates if they came to Britain.
He said it was over-simplistic to assume that control orders would be appropriate or even lawful against people simply because they had been detained elsewhere.