Britain's intelligence agencies have been ordered to destroy information they got illegally by spying on confidential communications between a Libyan dissident and his lawyers, a watchdog has ruled.

GCHQ, the Government’s listening post, unlawfully intercepted telephone calls and emails from Sami al-Saadi – even though the legally-privileged material was protected by strict rules.

In a landmark ruling yesterday, the Investigatory Powers Tribunal (IPT) the only judicial body with the power to investigate the security services, ordered spies to destroy two copies of the sensitive eavesdropped communications.

Government Communication Headquarters (GCHQ) in Cheltenham. The Government’s listening post unlawfully intercepted telephone calls and emails from Sami al-Saadi, it has been ruled

GCHQ confessed it intercepted confidential conversations between Mr al-Saadi (pictured) and his lawyer

Mr al-Saadi has fought a legal battle against the British Government claiming it was complicit in kidnapping him and sending him back to his homeland to be tortured by the Gaddafi regime.

But the IPT said the dissident should not receive compensation for breaching Article 8 of the European Convention on Human Rights, the right to private and family life.

Intelligence chiefs sparked fury in February by admitting that they had unlawfully listened in to lawyer-client communications even though they are generally covered by 'legal professional privilege' to protect privacy and the right to a free trial.

Now GCHQ has confessed it intercepted confidential conversations between Mr al-Saadi and his lawyer. The IPT rejected similar claims by other Libyan claimants including Abdel Hakim Belhadj, another prominent anti-Gaddafi campaigner.

Britain has already paid Mr al-Saadi £2.2million compensation after he took legal action over MI6's role in his abduction, imprisonment and mistreatment by Libya. Mr Belhadj is currently suing the UK.

It has not been disclosed what type of material the two documents contained or when they were recorded but they are likely to be transcripts of telephone conversations secretly intercepted by GCHQ.

Last night Mr al-Saadi said: ‘I had always been told I had a right to communicate with my lawyers about this torture case in private, but that clearly wasn’t the case.

Tony Blair with former Libyan dictator Muammar Gaddafi. Mr al-Saadi has fought a legal battle against the British Government claiming it was complicit in kidnapping him and sending him back to his homeland to be tortured by the Gaddafi regime

‘This was illegal behaviour and I hope now that the right to take legal advice in private will never be ignored again.’

Richard Stein, his solicitor, said: ‘GCHQ unlawfully spied on privileged legal communications for years, and the secret oversight mechanisms failed to stop it.

‘This marks the end of GCHQ’s standard boilerplate response that its activities are lawful, necessary and proportionate. I am sure that the unlawful behaviour we have uncovered in this case is the tip of the iceberg.’

Cori Crider, of legal charity Reprieve, which represented the families, said: ‘GCHQ spied on privileged legal communications, in a case where they were being sued by a rendition and torture victim.

‘We are pleased that one man has finally beaten the security services in this secretive tribunal. But this kind of illegal snooping makes phone hacking look like child’s play, because it rigs the whole justice game in the government’s favour.’

The IPT's ruling said that two documents involving al-Saadi ‘contained material subject to the legal professional privilege… held by GCHQ’.

It added: ‘In order to emphasise the importance of the protection of legally privileged material the tribunal has required GCHQ to give an undertaking that the parts of the documents containing legally privileged information will be destroyed or deleted so as to render such information inaccessible to the agency in the future.’

The IPT rejected similar claims to Mr al Saadi's made by other Libyan claimants, including Abdel Hakim Belhadj (pictured), another prominent anti-Gaddafi campaigner

Mr al Saadi was kidnapped with his family in Hong Kong in 2004, and tortured after his rendition to Libya following what he claims was a tip-off by MI6.

He was eventually freed and returned to Britain where he began legal action against the Government. In December 2012 Britain agreed to pay Mr al Saadi £2 million in compensation in an out of court settlement.

However, the Government did not admit liability for the rendition to Libya in 2004, nor confirm that MI6 was involved.

Mr Belhadj is currently suing the UK over the same scandal. He insists he was tortured during six horrific years in jail, including being beaten and hung from hooks on the walls.

The former Islamist rebel, now a prominent politician in Tripoli, has spurned a seven-figure settlement and wants an admission of guilt and an apology from Britain.

Last October, the Court of Appeal heard claims that the British Government under Tony Blair was complicit in the arrest of Mr Belhadj and his wife at Bangkok airport in 2004.

His legal battle raises the extraordinary prospect of Mr Straw and Sir Mark Allen, a former senior MI6 spy, being hauled before the courts to give evidence.