This article is the subject of a legal complaint from Sheikh Hamad bin Jassim bin Jaber al-Thani.

A former prime minister of Qatar is attempting to use his UK diplomatic immunity to halt a court case in London where he is accused of being responsible for the alleged torture of a British citizen.

Fawaz al-Attiya, a UK national born in London, claims that Qatari agents acting on behalf of Sheikh Hamad bin Jassim bin Jaber al-Thani falsely imprisoned him in Doha for 15 months and subjected him to conditions amounting to torture. Attiya claims he was kept in solitary confinement, deprived of sleep and only let out in handcuffs to be interrogated.

Hamad bin Jassim, sometimes known as “HBJ”, denies the allegations and court documents state that he plans to use diplomatic immunity to challenge the London high court’s jurisdiction at a hearing due to be held later this week.

Hamad bin Jassim, who has an estimated personal net worth of over $12bn (£7.8bn), stood down as Qatar’s prime minister and foreign minister in June 2013. The billionaire was also a key figure in the Qatar Investment Authority, a $200bn sovereign wealth fund, from 2000 until 2013 overseeing a series of high-profile investments in Harrods, the Shard and London’s Olympic Village.

In November 2014 he appeared on the UK’s Foreign and Commonwealth Office diplomatic list as a “minister-counsellor” serving in the London embassy and has remained on the diplomatic list ever since.

When asked about his diplomatic activities, Hamad bin Jassim’s lawyers have said his role in London includes “developing ... the very close relationship which exists between the state of Qatar and the UK, with particular emphasis and responsibility for the promotion and development of economic relations”.

In documents lodged in Companies House, the former Qatari PM lists his address as Al Wajba palace in Doha.

Diplomats enjoy legal immunity in Britain under the Vienna convention of 1961. A FCO spokeswoman said: “We cannot comment on court matters or individual diplomats.”

The UK court case revolves around a property dispute between Hamad bin Jassim and Attiya, a British citizen, whose great-grandfather was an emir of Qatar. Between 1996 and 1998 Attiya served as the Gulf state’s official spokesperson.

A claim form submitted to court by Attiya’s lawyers – Imran Khan and Partners – states that in 1997 Hamad bin Jassim made an offer to buy 20,000 square metres of valuable land from Attiya in the al-Rayyan area in west Doha. The court document does not state the value of the offer.

Attiya claims he refused the offer because it fell below his valuation and that this angered Hamad bin Jassim, which led to him leaving his job as Qatar’s spokesperson. He alleges his land was then seized by Hamad bin Jassim and that he was subjected to “increasing harassment, threats, and surveillance”.

The alleged feud lasted a decade. In late 2007 Attiya travelled to neighbouring Dubai for business where he claims HBJ tried to have him arrested.

“The claimant [Attiya] was told that the defendant [HBJ] was furious with him and that he planned to have him abducted from his home in Dubai,” read the claim form filed in London. Attiya was not arrested but in June 2008 he moved to Saudi Arabia where he claimed to feel safer.

At this time a series of legal cases were filed against Attiya in Qatar, including that he had leaked state secrets while serving in public office. The court documents claim that on 25 October 2009 Attiya was “forcibly taken from Saudi Arabia to Qatar”.

The document does not provide any further detail about this incident, but does go on to say that from this date until January 2011 Attiya was held in various detention centres across Qatar. It is understood that Hamad bin Jassim’s lawyers accept the incarceration took place but say it was lawful on the basis of state action which the former PM did not direct.

Attiya says he was “detained incommunicado” and “had no contact with other people except the guards”. “He was allowed out of his cell only for the purpose of interrogation and always in handcuffs,” the document states.

Attiya claims that while in prison has was prevented proper access to food, water, and sunlight. He also alleges that he was subject to sleep deprivation. “Long periods of time sitting on the cold tiled floor led to severe body pains ... Poor circulation led to his whole left hand and lower arm turning blue and becoming very numb.”

At one point Qatar’s assistant attorney general is claimed to have “stated that [Attiya] could be kept in precautionary detention pre-trial up to half of the remainder of his life”.

Attiya says he was told by Qatar’s assistant attorney that “he was being detained at the behest of the prime minister [Hamad bin Jassim], that there was no intention to release him and that any attempt to secure release through securing a court order ... would either be prevented or any such order would not be carried out”.

After a failed attempt to escape his cell, Attiya was released from prison, in late January 2011. He says he was told that he had been freed “on instruction of the crown prince of Qatar”. At that time the crown prince was Sheikh Tamim bin Hamad al-Thani, who since June 2013 has served as the country’s emir.

The cases against Attiya were formally dropped on 23 June 2011. However, Hamad bin Jassim is then alleged to have filed another case against him claiming that he had forged a cheque worth more than 3 million riyals (£520,000). Attiya denies that he ever wrote the cheque.

The court document states that this case was also dropped after intervention by the crown prince. Attiya now lives comfortably in a farmhouse near London but says he has been left severely out of pocket and is seeking compensation for the land he lost in Qatar.

Hamad bin Jassim’s lawyers did not respond to request for comment at the time of publication. The case between Attiya and Hamad bin Jassim continues.

After publication, lawyers for Hamad bin Jassim said that their client denied the allegations, which they described as “a combination of distortion, exaggeration and fabrication”. They said that the allegations were “wholly unsustainable on grounds relating to sovereign and diplomatic immunity, as well as on the demonstrable facts of the case.”

In a lengthy statement, Hamad bin Jassim’s lawyers said that the expropriation of property in the Al –Rayyan area was lawful and “an act of the government of Qatar” for which Fawaz al-Attiya was “better compensated” than any other individual. “By way of example, a number of (Attiya’s) cousins received much less”.

Describing claims of “unlawful rendition” as “nonsensical,” Hamad bin Jassim’s lawyers say Attiya was extradited from Saudi Arabia after a “warrant was issued for (his) arrest by the Attorney General of Qatar… (who) is accountable to the emir and not the prime minister.”

Attiya, says Hamad bin Jassim’s lawyers, was placed in the custody of the appropriate authorities and he was treated “in the manner that accorded fully with Qatari and international law”. It was “inconceivable”, they said, that Attiya was subjected to any form of “torture, coercion, threats or mistreatment”. Such acts, the lawyers said, are prohibited in Qatar.

Hamad bin Jassim also wanted to make it clear that he “had no involvement” in Attiya’s prosecution in relation to monies owed to the Qatar National Bank, a claim which involved a cheque worth more than 3 million riyals (£520,000).

This article was amended on 19 October 2015 to correct the sterling conversion of $12bn.