Saudi Arabia has been accused of launching a sophisticated hacking attack against a prominent dissident in London who is allegedly living under police protection, according to a letter of claim that has been sent to the kingdom and seen by the Guardian.

The letter of claim, which was delivered to the Saudi embassy in London on Tuesday, was sent on behalf of the Saudi satirist Ghanem Almasarir, and alleges he was targeted by Saudi Arabia with malware developed by the NSO Group, the controversial Israeli surveillance company.

NSO’s Pegasus software has allegedly been used by Saudi Arabia and other governments to target journalists, human rights campaigners and political activists.

A report earlier this month found that the spyware had previously taken advantage of a glitch on WhatsApp to try to gain access to a phone used by a British lawyer involved in a civil case against NSO Group.

According to the letter of claim, Almasarir received suspicious text messages in June 2018. These were tracked by independent experts to a Pegasus operator who was “focused on Saudi Arabia” and were linked to a separate attack against another Saudi critic.

The letter of claim alleges that certain indicators on Almasarir’s two Apple iPhones, coupled with the fact that he had clicked on corrupt weblinks sent to him, as well as Saudi Arabia’s widely reported use of Pegasus, led to the “inevitable conclusion” that the kingdom was responsible for sending Almasarir the texts and for the infection of his devices.

“A vast amount of Mr Almasarir’s private information was stored and communicated on his iPhones … This included information relating to his personal life, his family, his relationships, his health, his finances, and private matters relating to his work promoting human rights in Saudi Arabia,” the letter of claim said.

The Saudi government has six weeks to respond to the letter of claim, which a lawyer for Almasarir said could give the Saudis a chance to settle before the allegations are formally taken to court.

“We use the space of freedom in the west, in Europe and America and the Saudis want to take this freedom away from us by making me and others feeling like we are living in prison,” Almasarir told the Guardian.

The allegations rely in part on analysis conducted by Citizen Lab at the University of Toronto’s Munk School, a research laboratory that focuses on information security and has closely tracked NSO and Pegasus.

The Saudi embassy in London and a spokesman for the Saudi embassy in Washington did not reply to requests for comment on the allegations contained in the letter of claim.

NSO has previously defended its work. After the revelations about its alleged exploitation of a WhatsApp vulnerability to access users’ devices, it said it licensed and authorised governments to use its technology to fight “crime and terror”, but did not operate the system or determine how the technology was used by authorised government agencies.

The new allegation comes as other critics of the Saudi government – one in Norway and one in Canada – have been warned by local authorities that unspecified threats have been made against them by the kingdom. Saudi Arabia has declined to comment on previous reports of alleged threats to the dissidents.

Saudi Arabia faced international condemnation after the murder last October of the Washington Post journalist Jamal Khashoggi, a critic who lived in the US and was killed and dismembered inside the Saudi consulate in Istanbul. US intelligence agencies have concluded with a medium to high degree of confidence that the killing was ordered by the Saudi crown prince, Mohammed bin Salman. A close confidante of the prince, Saud al-Qahtani, is under sanctions in the US for his alleged role in planning the murder.

The Almasarir claim, as well as emerging reports of other alleged threats, suggest a new campaign is potentially being waged by the Saudi government against critics abroad.

Almasarir, who has lived in London since 2003, is known as a sharp-tongued critic of the Saudi royal family with more than 400,000 followers on Twitter. He has said his YouTube channel, the Ghanem Show, which has received 230m views, is meant to increase awareness of Saudi’s human rights record and poke fun at the royal family.

The letter of claim describes allegations of intimidation, including Twitter campaigns against him that are said to have promoted violence, as well as attempts over the years to lure him back to Saudi Arabia or to travel outside the UK.

The Saudi embassies in London and Washington did not respond to allegations of an intimidation campaign.

According to the letter of claim, Almasarir has been living under police protection in London since 31 October.

The Guardian could not independently verify Almasarir’s account.

The Metropolitan police in London said it could not confirm or deny whether Almasarir was under police protection because it does not discuss matters of security.

A Canadian-based Saudi dissident, Omar Abdulaziz, has filed a lawsuit in Israel claiming NSO software was used to target his phone in 2018, at a time when he was in regular contact with Khashoggi. Abdulaziz has also recently been made aware by local Canadian security officials of a possible threat against him, according to a report in Time magazine that cited his friends and associates.

The NSO Group has said in response to the legal claim in Israel that its products were licensed for governments to “lawfully fight terrorism and crime” and that it did not tolerate “misuse” of its products.

The Guardian reported earlier this month that another critic, İyad el-Baghdadi, who has received asylum protection in Norway, was recently made aware of an unspecified threat against him emanating from Saudi Arabia. The warning was passed on by Norwegian authorities but originated with the US Central Intelligence Agency.

The letter of claim by Almasarir alleges he suffered personal injury as a result of Saudi Arabia’s alleged campaign against him. His law firm, Leigh Day, is seeking unspecified damages, a public apology from Saudi Arabia, full Saudi disclosure about individuals who acted on its behalf, a return of allegedly stolen information, and costs.