Former prime minister of Qatar at centre of court case brought by UK-born ex-colleague who alleges he was falsely imprisoned for 15 months

A former prime minister of Qatar has been accused of running a landmark London hotel and doing business deals worth hundreds of millions of pounds while claiming diplomatic immunity under the Vienna convention, which forbids commercial activity for personal profit.

Sheikh Hamad bin Jassim bin Jaber Al Thani is at the centre of a case brought in the English courts by Fawaz al-Attiya, a UK national born in London, who claims Qatari agents acting on the sheikh’s behalf 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. He also alleges that Thani, also known as HBJ, arranged for the confiscation of his property. Attiya is seeking damages of hundreds of millions of pounds for the loss of ancestral land.

Lawyers for HBJ said the allegations were “a combination of distortion, exaggeration and wholesale fabrication”. They said the former PM did not direct and had no involvement with Attiya’s incarceration.

No defence has yet been served by HBJ because his lawyers are challenging the jurisdiction of the court to deal with the claim, arguing that as a diplomat HBJ is entitled to immunity.

Facebook Twitter Pinterest Fawaz al-Attiya. Photograph: Social media

In his first interview about the case, Attiya claims that HBJ’s diplomatic credentials are a sham. “Can you imagine that the former PM could be a diplomat at the embassy?” he said.

He pointed out that in television interviews and in public HBJ had “frequently referred to himself as a private citizen, not as an official representing Qatar. Yet now he claims to be a diplomat at the embassy. Here he is saying I have diplomatic immunity. I am all powerful here. I will push around not only the Qatari government but the British government too.”

Attiya alleges that HBJ arranged for diplomatic status to shield himself from charges that he was responsible for the alleged torture of a British citizen.

Lawyers for HBJ dispute this, saying the appointment pre-dates the legal claim and that his position at the embassy is to “develop yet further the very close relationship that exists between Qatar and the UK, with particular emphasis on the promotion and development of economic relations”.

HBJ has an estimated personal net worth of more than £7.8bn and he was 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 June 2013 HBJ stepped down from the posts of prime minister, foreign minister and head of the Qatar Investment Authority after his ally, the Qatari emir, abdicated in favour of his son, Sheikh Tamim bin Hamad Al Thani.

Six months later HBJ appeared in London on the Foreign Office diplomatic list, eventually given the rank of “minister-counsellor”. The listing as a diplomat means he and his two wives and 15 children enjoy legal immunity in Britain under the Vienna convention of 1961.

The dispute over HBJ’s diplomatic status is due to come to court in January when the court will hear Attiya’s allegations that HBJ and his family have extensive business interests in the UK.

It is alleged that HBJ is a director and owner of the company that owns the five-star Intercontinental Park Lane in London, which was sold for a reported £400m in 2013. He is also said to be a director of Constellation hotel (OpCo) UK SA, whose stated business is the “running of an hotel business, management of the Intercontinental hotel Park Lane”.

HBJ’s defence team denied that this activity would be in contravention of the Vienna convention. In an email to the Guardian, HBJ’s lawyers said his diplomatic appointment was notified to the British government “and the latter was clearly satisfied that the requirements of the Vienna convention were met when it accepted and accredited my client as minister-counsellor”.

They added that a diplomat was not precluded from holding investments in a country they were stationed in. HBJ’s right-hand man in Doha, Fady Bakhos, said: “Constellation OpCo does not run or manage Intercontinental hotel. Constellation OpCo is a special purpose company [and] is a company registered in Luxembourg and it is registered as permanently established in England for tax purposes only.”

HBJ bought 80% of Heritage Oil, a London-listed exploration and production company, in June 2014. The “recommended cash offer” for Heritage Oil, dated 30 April 2014, states that the £924m stake would transfer to a wholly owned subsidiary of Al-Mirqab Capital, described as an “investment vehicle which is indirectly and beneficially owned by His Excellency Sheikh Hamad Bin Jassim Bin Jaber Al Thani and his family in a private capacity”.

However, HBJ’s lawyers said: “The acquisition was obviously a single transaction rather than a continuous activity. The mere fact the shares in Heritage Oil were listed in the UK prior to the acquisition is patently insufficient to mean that the activity in question took place in the receiving state.”

HBJ’s lawyers denied that he was a director or direct shareholder of any English trading company.

The upcoming court hearing will decide whether HBJ can prevent Attiya from taking civil legal action against him.

The case pits two former colleagues and rivals – each minor Arab nobility – against each other. Both are related to the founding father of Qatar, Jassim bin Mohammed Al Thani. Attiya is the first emir’s great-grandson, and HBJ is the founder’s grandnephew.

Almost two decades ago both men were once at the side of the then emir of Qatar, Hamad bin Khalifa Al Thani. Between 1996 and 1998 Attiya served as the Gulf state’s official spokesperson when HBJ, a decade older, was the foreign minister. However, the two fell out in 1998. Attiya’s family land was confiscated five years later.

HBJ’s lawyers said the land deal was lawful and “an act of the government of Qatar” for which Attiya had been “better compensated” than any other individual. “By way of example, a number of [Attiya’s] cousins received much less,” they said.

The resulting feud lasted more than a decade, and Attiya claims he was “forcibly taken from Saudi Arabia to Qatar” on 25 October 2009 and held in various detention centres until January 2011.

“If you had asked me when I came out about it all I would have broken down in tears. I was kept in solitary confinement for eight months where no one spoke to me, no visitors,” said Attiya. “I was being made to pay because I dared to speak out against [HBJ]. He wanted to make an example of me.”

Describing claims of unlawful rendition as “nonsensical”, HBJ’s lawyers say Attiya was extradited from Saudi Arabia after a warrant was issued for his arrest by the attorney general of Qatar, who they say is “accountable to the emir and not the prime minister”.