The Metropolitan Police are to investigate claims that British agents colluded in torture, Scotland Yard said today.
Officers are to investigate allegations by former Guantanamo Bay detainee Binyam Mohamed that MI5 officers were complicit in his torture.
The case was referred to police by the Attorney General, Baroness Scotland, earlier this year.
A Met spokesman said: "The Metropolitan Police Service (MPS) was invited by the Attorney General to investigate allegations surrounding the detention of Binyam Mohamed.
"The papers were reviewed by the MPS and the investigation accepted. A team of detectives, working to Deputy Assistant Commissioner Sue Akers, has now been selected and vetted to appropriate levels.
"As a result a criminal investigation has now begun.
"The inquiry team is in close liaison with the Crown Prosecution Service and will regularly consult them as the investigation moves forward.
"Inquiries will be conducted as expeditiously but thoroughly as possible and will follow the evidence to identify whether any offences have occurred."
Mohamed, 30, a former UK resident, was arrested in Pakistan in 2002.
He alleges that, during three months of detention, he was tortured by Pakistani agents and interrogated by the FBI and MI5.
He says he was then taken to Morocco after being subject to "extraordinary rendition" by the CIA with the explicit knowledge of the Security Service.
During further torture in Morocco, he says he became aware that his torturers were being fed questions and material from British intelligence agents.
Ethiopian-born Mohamed, who lived in London before his arrest, was then taken to Guantanamo Bay, where he stayed for four years before returning to this country on February 23.
Then-home secretary Jacqui Smith originally referred Mohamed's claims to Lady Scotland in October last year after they first surfaced in a High Court case brought by his lawyers.
In a statement made following his return to the UK, Mohamed said "the very worst moment came when I realised in Morocco that the people who were torturing me were receiving questions and materials from British intelligence".
Former shadow home secretary David Davis said: "Every step of this process has reinforced the fact that there is a serious case to answer on the torture of Binyam Mohamed and complicity in that of agents of the British Government.
"I hope that this investigation is carried out expeditiously so we can quickly draw to a conclusion the events of these last seven years.
"Nevertheless, this is not a substitute for a proper judicial inquiry into how our Government allowed itself to become complicit in torture, thereby undermining our moral standing in the struggle against terrorism and Islamic extremists."
Shami Chakrabarti, director of human rights group Liberty, said: "Having first made police complaints in 2005, we are hugely encouraged that a criminal inquiry into British complicity in torture has finally begun.
"We fear that the police will soon need to broaden their investigations into a number of other cases and that a broader judicial inquiry into the darkest aspects of the UK's war on terror is now unavoidable."