Claims that secret agents working for MI5 and MI6 watched and encouraged the torture of a second British resident held by the Americans must be investigated by the police, politicians, lawyers and human rights groups said last night.

Fresh allegations in the High Court this week will increase pressure on the Government to open a judicial inquiry into collusion between the CIA and British security services in torture during the "war on terror".

Shaker Aamer, 42, the final British resident in Guantanamo Bay, claims that like Binyam Mohamed, he was tortured in American custody in 2002.

On Thursday the Attorney General took the unprecedented step of calling in Scotland Yard to investigate allegations that MI5 collaborated in the alleged torture of Mr Mohamed. Mr Aamer's case is believed to be one of 15 similar new cases. In a legal claim against the Government to be lodged at the High Court this week, Mr Aamer's lawyers allege that MI5 and MI6 were complicit in the torture of Mr Aamer.

A claim letter sent to Government lawyers alleges: "UK intelligence services officers were present whilst Mr Aamer was beaten. They provided information and encouragement to his US torturers. They made no attempt to stop his ill-treatment or any enquiries into his well-being."

The British Government has requested the release of Mr Aamer from Guantanamo Bay, where he is on hunger strike. There are no terrorism charges against him.

Dominic Grieve, the shadow Home Secretary, last night called on the Government to investigate Mr Aamer's allegations. "These disturbing claims only strengthen the case for a judicial inquiry into allegations of British collusion in torture," he said. "Britain's reputation will be dragged through the mud if they go unanswered."

According to an MI5 senior manager, known as "Witness A", the Security Service was aware of transfers of prisoners from Pakistan to Afghanistan during this period. Witness A's evidence in the case of Binyam Mohamed, given last year but only released on Friday, suggests at least some of these transfers were believed to be unlawful.

Mr Aamer's lawyer Irène Nembhard, of Birnberg Peirce, is asking the British Government to disclose the identity of the US and UK agents involved in her client's interrogation, detention and torture.

Mr Aamer is married to a British citizen with whom he has four children, including a son he has never seen. He says he went to Afghanistan in 2001 to work with charities. He was sharing a house with Moazzam Begg, the British man released from Guantanamo Bay in 2005, when he was forced to flee the US-led invasion. He says he was held by an Afghan militant group, who passed him to the Northern Alliance who sold him to the Americans. The CIA arranged his detention in Afghanistan and transfer to Guantanamo Bay. Mr Aamer says an MI5 officer, who gave his name as "John", was present while he was tortured in Afghanistan in January 2002. "John" was the name given by Witness B, the MI5 agent who interrogated Binyam Mohamed.

Belfast Telegraph