Dozens of Al Jazeera journalists were allegedly hacked with the help of spyware developed by Israeli firm NSO Group, cyber-security researchers say.

Details of the alleged hack targeting 36 members of staff, including TV anchors and executives, have been published in a report by Citizen Lab at the University of Toronto.

It says a vulnerability in iPhone operating system software was used.

NSO Group has denied the allegation, saying it “lacks any evidence”.

Citizen Lab researchers say they concluded with “medium confidence” that two attackers who had spied on the phones of Al Jazeera journalists were doing so on behalf of the Saudi Arabian and UAE governments.

“The phones were compromised using an exploit chain that we call Kismet,” the researchers write.

In July 2020, Kismet was a “zero-day” attack – meaning Apple was supposedly unaware of the flaw – and it worked on at least iOS 13.5.1, and could hack Apple’s iPhone 11, the latest model at the time.

Citizen Lab was first alerted to potential spying activity on the journalists’ phones when contacted by Tamer Almisshal, an investigative filmmaker at Al Jazeera.

Mr Almisshal had expressed concerns that his iPhone had been hacked and so allowed Citizen Lab to monitor activity on the device.

“We noticed that on 19 July 2020, his phone visited a website that we had detected in our internet scanning as an Installation Server for NSO Group’s Pegasus spyware, which is used in the process of infecting a target with Pegasus,” the Citizen Lab researchers allege in their report.

Hitting back at the allegations, a spokesman for NSO Group said: “This memo is based once again on speculation and lacks any evidence supporting a connection to NSO.”

He added that the firm provided software to governments, who used it tackle serious organised crime and terrorism, and did not operate the software itself.

NSO Group would continue “to work tirelessly to make the world a safer place”, he added.