Q and A: UK rendition allegations
What is the key allegation that has been levelled at the British security services?
It's about collusion in ill treatment and torture. In the aftermath of 9/11, the US and others invaded Afghanistan in an attempt to crush al-Qaeda. The British government was among those aiding the US mission because of a mutual interest in targeting Islamist extremism. Some of the individuals seized by the Americans - or sometimes by the security forces of other countries - had connections to the UK.
The UK's two security agencies, MI5 and MI6, were battling to grasp the extent of the threat posed by al-Qaeda and knew some detainees held abroad had the key to plots at home. That meant British officers needed to meet and interview anyone who had a connection to the UK.
So what's that to do with allegations of collusion?
In the years that followed, it emerged that some of the detainees held in foreign prisons had been abused or tortured. The question was whether British officials aided and abetted that abuse.
The most well-known case involves Binyam Mohamed, a British resident who was eventually released from Guantanamo Bay.
Mr Mohamed was detained in Pakistan in 2002 and handed over to US authorities He was moved to Morocco and Afghanistan before being finally detained at Guantanamo Bay. Mr Mohamed says that he was abused in Pakistan and tortured in Morocco.
He won a case at the Court of Appeal in London which revealed what the US had told the UK about the detainee's ill treatment. Mr Mohamed accused the British security services of colluding in his mistreatment and doing nothing to stop it.
What does the law say about torture?
Torture is banned under British law and international convention. The UK also has a long-standing prohibition on lesser forms of abuse, relating to interrogation practices that were outlawed during The Troubles in Northern Ireland.
Did British officials torture or abuse anyone?
There are no allegations that British officials broke the law in this way. The allegation is that they either turned a blind eye or did not do enough to stop foreign security agencies abusing detainees. Proving an offence in this area is a very complex business, the key offences being aiding and abetting or misconduct in a public office.
So who has been investigated?
An MI5 officer known only as Witness B was investigated over alleged complicity in Mr Mohamed's ill treatment in Pakistan.
In November 2010, the Crown Prosecution Service announced that there was insufficient evidence to charge the officer in relation to that interview.
The CPS then looked at whether the British had acted unlawfully by supplying the Americans with questions to be used during Mr Mohamed's interrogations in Morocco. The CPS found that there was evidence to show that MI5 staff supplied questions - but did not know where he was being held or what was happening to him. In other words, there was insufficient evidence to charge anyone with assisting his torture.
The CPS statement stresses that its decision not to charge does not undermine Mr Mohamed's account of what happened to him.
The third element of the investigation relates to an MI6 officer at Bagram Air Base in Afghanistan. The CPS says there was a "discrete incident" involving someone being held at the base and being interviewed by the officer in January 2002. We don't know what happened - but the Metropolitan Police has not been able to get an account from the detainee and possible eye-witnesses. The CPS decided it could not charge the officer.
So that's the end of the story then?
No. During the fall of Gaddafi, documents have emerged which suggest that the British officials were allegedly involved in the transfer of two of the former dictator's opponents back to their home country, where they were then tortured.
Both men have threatened to sue the UK for its alleged role in their treatment. It's these allegations that are at the centre of the new investigation.
Further allegations have been made by Shaker Aamer - the last remaining British resident at Guantanamo Bay. He was previously held at Bagram.
Two more men, both jailed for life in the UK for serious terrorism offences, alleged that they were tortured in Pakistan at the behest of the British.
Is there going to be a public inquiry?
There is going to be a limited independent inquiry led by retired judge Sir Peter Gibson. But the men who were held in Guantanamo Bay and other locations have already said they will boycott the hearings because too many of the hearings will be in secret.
However, it is no longer clear when the inquiry can start, given that police must investigate the new allegations.