Cori Crider, legal director of human rights organisation Reprieve, said: 'Although the British and Americans are the chief organisers of this business with the Libyans, the Hong Kong security services were prepared to turn a blind eye to what was going on in their backyard.

'I absolutely think it's an important part of the case.'

The Hong Kong government and the security bureau have refused to comment in the two months since the case emerged, despite repeated requests. Beijing has acknowledged the rendition but said Hong Kong had complete authority to decide on who entered or left the territory.

Details of Hong Kong's role in the rendition in 2004 remain sketchy.

But according to papers discovered during the chaos of the Libyan revolution, Saadi was tricked into travelling to the city from the mainland before eventually being put on a plane to his homeland.

His legal representatives are accusing Mark Allen, former head of the UK intelligence agency MI6, and Mussa Kussa, Gaddafi's former security chief, of conspiracy to torture.

What is known about the case comes from Saadi - who was branded a jihadist by the Gaddafi regime - and a secret CIA fax found in Kussa's abandoned Tripoli office. Saadi fled Libya in 1988 and was given permission to stay in the UK in 1994. He then spent a year in Guangzhou but decided he wanted to return to the UK.

He claims British officials said they could arrange his return if he and his family went to their embassy in Hong Kong.

But after he arrived at Chek Lap Kok with his family of five - the youngest member a six-year old girl - they were all detained, then sent back to Tripoli 13 days later.

Saadi says he was imprisoned from 2004 until last year, and was whipped, beaten and electrocuted by Libyan interrogators. He claims he was also questioned by MI6 and the CIA. He spent his last year in prison on death row.

Saadi was freed from the Abu Salim jail on August 23 this year after rebels stormed the site.

He was carried out, weighing just 44kg. But the fax sent from the CIA to Tripoli indicates that Hong Kong demanded an assurance Saadi and his family would not be tortured or killed. Stanley Ying Yiu-hong, the then permanent secretary for security, is singled out as a contact.

It is Ying, along with other members of the Hong Kong government, who Saadi's lawyers want to talk to.

It is understood their questions will focus on whether Ying was the key decision-maker on the rendition, or whether other officials were involved. Other questions will concern the request for an assurance that the Saadi family would be treated humanely.

The lawyers will try to determine who made the request and whether they were convinced it would be upheld once Saadi and his family were back on Libyan soil.

Crider said: 'There's no colourable argument that people think that Gaddafi's people, Mussa Kussa and the rest, were going to do anything other than torture folks when they get sent back. They have a well and richly deserved reputation.'

The Metropolitan Police in London confirmed they had received the criminal complaint and were considering a formal investigation.

The criminal case follows a civil action against the British government which was launched by Saadi last month. It claims damages for complicity in torture, misfeasance in public office and negligence.

Under British legal procedures the government has six months to accept or deny the claim.

This latest case against the British intelligence services comes after British Foreign Secretary William Hague hailed their work in Libya.