Ex-Foreign Secretary Jack Straw faces being sued over allegations of abduction and torture brought by a former Libyan dissident.
Abdul-Hakim Belhaj claims MI6, which Mr Straw was then responsible for, helped the US kidnap him in Asia in 2004 to return him and his wife to Tripoli.
The Supreme Court backed a Court of Appeal ruling allowing his action.
Mr Straw – among several parties in the case – rejects claims that he had been aware of the rendition.
Lawyers for Mr Belhaj, 50, say he is determined to sue unless he receives an apology and a token £1 in damages.
Mr Belhaj, one of the leaders of the uprising against former Libyan leader Colonel Muammar Gaddafi, says he was abducted – along with his pregnant wife, Fatima Boudchar – as he was about to fly to London to claim asylum.
After being returned to Libya, he spent six years in jail, while his wife was released shortly before giving birth.
The allegations against the UK security services
As well as former Labour MP Mr Straw, the case brought by Mr Belhaj and his wife is against former senior MI6 official Sir Mark Allen, the UK security services, the Foreign Office and the Home Office – all have denied liability.
The Supreme Court ruling examined whether UK courts should intervene in acts of a foreign state and if claims brought against it could be barred by state immunity.
In the ruling, the judges said the allegations were so serious they had to be heard before a British court because, if not, they may never be heard anywhere else in the world.
In a statement, Mr Straw said: “At no stage so far have the merits of the applicant’s case been tested before any court…
“As foreign secretary I acted at all times in a manner which was fully consistent with my legal duties, and with national and international law.
“I was never in any way complicit in the unlawful rendition or detention of anyone by other states.”
The justices also gave the green light to a related but separate case brought by a Pakistani man who says the UK illegally handed him over to US forces in Iraq.
A government spokeswoman said it would consider the detail and implications of both rulings but it would be inappropriate to comment further because of the ongoing legal proceedings.
Magna Carta
Mr Belhaj, who was once regarded by Western intelligence services as a terrorism suspect, alleges he was questioned by British intelligence officers during his detention in Libya.
His damages action is based on documents uncovered in Tripoli following the fall of Colonel Gaddafi.
The papers show that, in 2004, MI6 communicated with the regime over the dissident’s fate after he had fled the country.
According to the papers, MI6 tipped off the Libyan regime and the couple were seized in Bangkok by US secret services.
In one of the documents, Sir Mark Allen, MI6’s former head of counter-terrorism, tells a Libyan intelligence official that Mr Belhaj’s capture had been possible only thanks to British intelligence.
The government has neither confirmed nor denied whether the documents are genuine.
In its unanimous judgement, the Supreme Court justices dismissed the government’s attempts to stop the case going ahead to trial, saying that the Magna Carta was on the side of Mr Belhaj and his Moroccan-born wife.
Lord Mance said: “English law recognises the existence of fundamental rights, some long-standing, others more recently developed.
“Among the most long-standing and fundamental are those represented by the Magna Carta which reads, ‘No free man shall be taken, or imprisoned… excepting by legal judgement of his peers of the land’.”
Lawyers for the government had argued Mr Belhaj and Ms Boudchar could not sue because of a long-standing legal principle that prevents British courts examining the actions of other nations carried out in their own jurisdictions.
Prosecutors last year said there was insufficient evidence to charge anyone from MI6 with a criminal offence.
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