MI6 prefers not to spell out the limits of its powers but sources have made it clear that the service does not have a “licence to kill.”
One security source told the Daily Telegraph that the service does not get involved in extra judicial killing or assassinations.
The source explained that MI6 officers are not allowed to do anything abroad that would be illegal in Britain, nor ask anyone to do it on their behalf.
Where they have concerns about an operation they are obliged to abort the mission or seek “reliable caveats or assurances.”
Section seven of the Intelligence Services Act 1994 allows the Foreign Secretary to make exceptions but they are normally limited to specific acts such as theft, payment to an agent and bribery.
The mission of the service is seen as recruiting and running agents and collecting intelligence rather than running operations against enemies abroad.
Adherence to human rights and the involvement of lawyers has become a pervasive element in the work of both MI5 and MI6, sources say.
Some old intelligence hands feel their work has been stifled by the need to seek legal approval at every turn.
The CIA on the other hand runs a large-scale, and still classified, programme in the skies over Pakistan and Yemen to assassinate militants using Hellfire missiles fired from unmanned drones.
US intelligence officers believe the programme has “removed a lot of senior leaders from the battlefield” making communication, training and planning difficult, if not impossible.
The CIA operate with the cover of a “lethal finding”, approved by Congress, to launch covert operations against al-Qaeda that could result in fatalities.
But the White House has begun to acknowledge, if only behind closed doors, that the decision to assassinate terrorists needs some kind of framework to ensure that it does not become the arbitrary decision of whoever is in power.
Sources have made clear that Britain is not part of the US drone programme but the government is still facing a legal challenge into whether it shares intelligence which could be used by the US to target missiles.
The SAS is available for hostage rescue anywhere in the world but “kill or capture” missions remain effectively limited to Afghanistan.
Where special forces have been invited into countries fighting an insurgency such as Yemen, normally on “training missions”, they may be allowed to take part in joint missions with their hosts.
But such is the caution at the top of the Foreign Office and the Ministry of Defence that those forces are unlikely to be deployed on anything that resembles an “assassination.”
Public utterances from MI6 are extremely rare and insights into their ethics rarer still.
Sir Richard Dearlove, who retired as chief of MI6 in 2004, said that lethal force can be used by the service but only under extreme circumstances.
His comments were made in evidence to the inquest into the death of Diana, Princess of Wales.
Sir Richard said that legal provision was made for "lethal force" to be used with Government authorisation in times of “emergency or crisis which caused danger to the UK or its citizens.”
He added that the service was legally required to seek authorisation from the Foreign Secretary to carry out any operation which involved breaking the law.
Sir Richard said he had never heard of lethal force being exercised in his 38-year career.
He said a plan to kill a Serb leader who had been involved in ethnic cleansing was "killed stone dead" because the proposal was "out of touch with service practice, service ethos and it was not a proposal to which consideration would be given.”
Allegations that MI6 was involved in a plot to kill Libyan leader Colonel Gaddafi were simply "not true", he added.
Sir John Scarlett, who retired as chief of MI6 in 2009, said in an interview with the BBC that there were “limits” to what officers were allowed to do to secure information but added: “I’d prefer not to get into all the specifics about the particular techniques.”
Asked about the fabled “licence to kill” he said: “The licence to kill issue? No we do not. We do not have licence to kill.”
Sir John Sawers, the current chief of MI6, has said that the service is prepared to let terrorist activity go ahead in order to stay within British and international law.
His comments were made in reference to torture rather than killing, but he told his audience in the only speech he has given: “If we know or believe action by us will lead to torture taking place, we’re required by UK and international law to avoid that action. And we do, even though that allows the terrorist activity to go ahead.”
Six weeks after Awlaki was killed, William Hague, the Foreign Secretary, said that intelligence work “throws up some of the most difficult ethical and legal questions that I encounter as Foreign Secretary.”
Mr Hague, who oversees MI6 and would have to give the go-ahead for any assassination plot, added: “As a Government we understand how important it is that that we not only uphold our values and international law, but that we are seen to do so.”
However the law remains poorly defined and human rights lawyers are currently seeking a judicial review to get it more clearly outlined.
A Pakistani citizen is using legal aid to seek a ban on Britain passing targeting information to the Americans claiming it constitutes “encouraging or assisting murder” under the Serious Crime Act 2007 and that it could also amount to was crimes or crimes against humanity.
Martin Chamblerlain, acting for Noor Khan, told the High Court on October 23: “We say the offence of murder is triable in England where the defendant is a subject of Her Majesty even where the killing takes place abroad.”
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