A Lloyd's insurance syndicate has begun a landmark legal case against Saudi Arabia, accusing the kingdom of indirectly funding al-Qa'ida and demanding the repayment of £136m it paid out to victims of the 9/11 attacks.

The Brighton-based Lloyd's 3500 syndicate, which paid $215m compensation to companies and individuals involved, alleges that the oil-rich Middle Eastern superpower bears primary responsibility for the atrocity because al-Qa'ida was supported by banks and charities acting as "agents and alter egos" for the Saudi state.

The detailed case, which names a number of prominent Saudi charities and banks as well as a leading member of the al-Saud royal family, will cause embarrassment to the Saudi government, which has long denied claims that Osama bin Laden's organisation received official financial and practical support from his native country.

Outlined in a 156-page document filed in western Pennsylvania, where United Airlines flight 93 crashed on 9/11, the claim suggests that the nine defendants "knowingly" provided resources, including funding, to al-Qa'ida in the years before the attack and encouraged anti-Western sentiment which increased support for the terror group.

The legal claim states: "Absent the sponsorship of al-Qa'ida's material sponsors and supporters, including the defendants named therein, al-Qa'ida would not have possessed the capacity to conceive, plan and execute the 11 September attacks. The success of al-Qa'ida's agenda, including the 11 September attacks themselves, has been made possible by the lavish sponsorship al-Qa'ida has received from its material sponsors and supporters over more than a decade leading up to 11 September 2001."

The Lloyd's syndicate is known as a "run-off", meaning that it does not accept new premiums on the Lloyd's of London insurance market and instead deals with historic claims. In the wake of the 9/11 attacks, its members settled a raft of multimillion-pound claims from affected businesses, including airlines, airports and security companies, as well as injured individuals and relatives of those killed.

Its complaint, which quotes heavily from US diplomatic cables released by WikiLeaks detailing investigations by the US authorities into al-Qa'ida, attempts to establish funding links between some Saudi charities, and the terror group, and implicate the Saudi government in that funding through its support of the charities.

The case singles out the activities of a charity, the Saudi Joint Relief Committee for Kosovo and Chechnya (SJRC), which was alleged by UN officials to have been used as a cover by several al-Qa'ida operatives, including two men who acted as directors of the charity. It is alleged that at the time the SJRC was under the control of Prince Naif bin Abdul Aziz al-Saud, half-brother of King Abdullah and the long-standing Saudi Interior minister. The claim states: "Between 1998 and 2000, the Kingdom of Saudi Arabia, through the SJRC, diverted more than $74m to al-Qa'ida members and loyalists affiliated with SJRC bureaus. Throughout this time, the Committee was under the supervision and control of Saudi Interior Minister Prince Naif bin Abdul Aziz."

The Saudi embassies in London and Washington did not respond to requests from The Independent for a response to the allegations in the claim. The 9/11 Commission, America's official report on the attacks, found that there was no evidence that the Saudi government or senior Saudi officials individually funded al-Qa'ida.

Diplomatic cables obtained by WikiLeaks showed that American officials remained concerned that the Saudi authorities were not doing enough to stop money being passed to the terror group by Saudi citizens.