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Prime Minister David Cameron has said a "judge-led" inquiry will look at claims that UK security services were complicit in the torture of terror suspects.

What will this inquiry set out to do?

Prime Minister David Cameron says the inquiry will be headed by Sir Peter Gibson, a former Court of Appeal judge and the watchdog charged with overseeing how MI5 bugs suspects.

In his letter to Sir Peter, the prime minister said: "The purpose of this inquiry is to examine whether, and if so to what extent, the UK government and its intelligence agencies were involved in improper treatment of detainees held by other countries in counter-terrorism operations overseas, or were aware of improper treatment of detainees in operations in which the UK was involved."

What will it examine?

Mr Cameron has asked the inquiry to have a "particular focus" on the immediate aftermath of 9/11 and cases involving the detention of UK nationals and residents in Guantanamo Bay. But the inquiry will be free to pursue wider angles because some of the most serious claims of alleged complicity relate to incidents in Pakistan.

So what are the key cases?

There are approximately a dozen cases that we know of. The story of Binyam Mohamed is the most well-known.

He was held in Pakistan in 2002 before US agencies moved him to Morocco, where he was severely tortured. He ended up in Guantanamo Bay and was returned to the UK in early 2009.

It then emerged that a British intelligence officer visited him in detention in Pakistan and that the CIA later told London what mistreatment the suspect had suffered.

So how will the inquiry work?

The prime minister says he hopes the inquiry can start before the end of the year and take no longer than 12 months - but it cannot start until criminal investigations reach a conclusion.

Detectives are looking at the actions of one MI5 officer who was linked to Binyam Mohamed's case. MI6 chiefs separately referred one of their officers to the police in relation to a different case. Neither of these investigations has been completed.

Secondly, a string of former Guantanamo detainees are trying to sue the government over complicity in their detention.

The government now wants to mediate with these men and possibly offer them compensation.

If and when that mediation makes progress, the inquiry is likely to begin its work.

Will the inquiry be in public?

No prime minister would ever launch an inquiry that would expose how the intelligence and security services work because it would risk damaging their capability to deal with future threats. So don't expect that any intelligence assessments will be made public.

But the inquiry will be able to hear in public from those who are making the allegations against the security services.

There will be a report - but what is made public will be a decision for the prime minister.

In essence, this means that nothing will be published that he believes would compromise the secret methods of MI5 and MI6.

This could mean that only some of its findings will be made public if the prime minister concludes that publishing all of them would be damaging to national security.

Will officers face prosecution if they admit to complicity?

Attorney General Dominic Grieve has agreed that nobody can have their evidence used against them in a prosecution unless they are specifically accused of giving false evidence, be it in public or secret sessions.

This arrangement is similar to the undertaking that was given to the soldiers who were called to give evidence to the Bloody Sunday Inquiry.

Will the inquiry look at wider allegations of complicity?

Mr Cameron told MPs that the inquiry would be free to look at allegations that agencies were complicit in CIA operations to secretly fly mistreated suspects from country to country, a practice known as "extraordinary rendition".

This could include examining how and why some detainees were moved through or held at Diego Garcia, the British Indian Ocean Territory that hosts a US air base.

What's been the reaction?

Gareth Peirce is a solicitor who represents eight former Guantanamo Bay detainees and she said that the public interest lay in openness.

"A true picture of the involvement of the state as a whole, at every level, is of far greater potential importance than the culpability of any individual employee of the state," said Ms Peirce.

"What is to come depends entirely upon the extent of openness, honesty and completeness that follows. Without that, any process, whatever its intention, must be doomed."

Legal charity Reprieve, which fought the court battle to reveal what London knew about Binyam Mohamed's treatment, said that the government must publish old rules on torture and interrogations overseas.

Clare Algar, director of the charity, said: "It is essential that this inquiry look at the official policy and rules under which British agents were operating.

"A new version of the rules has [now] been published, but the fact that the government refuses to publish the old rules suggests that they were embarrassing if not illegal. The inquiry must address this, in public, if we are to learn from our mistakes."