Q&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 or alternatively to assist foreign security agencies to apprehend others.
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 or, at the least, had been told what was going on and did nothing to intervene.
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 long-standing English common 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. The European Convention on Human Rights also prohibits torture - but British law predates that.
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 would be 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 Libya's Colonel Gaddafi, documents emerged which suggested that the British officials were allegedly involved in the transfer of two of the former dictator's opponents, and their families, back to their home country, where they were then tortured. One of those men settled - but the other, Abdul Hakim Belhaj, pressed on. He and his wife say the UK was complicit in their rendition from the Far East to Libya. The family have refused to accept out-of-court settlements, saying they will only settle for a token £3 plus an admission of wrongdoing and an apology.
In October 2014, the Court of Appeal ruled that their case should be heard in and English court, despite the government's resistance, because of the grave allegations levelled against officials and the former Foreign Secretary Jack Straw.
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 was going to be a limited independent inquiry led by retired judge Sir Peter Gibson - but that was binned partly because of the ongoing legal action relating to Syria. The government has promised that the Intelligence and Security Committee in Parliament will look into the allegations - and it has now asked for detailed submissions as part of that work.