UK opposition politicians called for an investigation Saturday after a newspaper reported that suspected Kremlin agents had hacked ex-Prime Minister Liz Truss's cell phone when she was foreign minister.

In an unconfirmed report, The Mail on Sunday cited unnamed security sources as saying that Truss's personal mobile phone had been hacked "by agents suspected of working for the Kremlin".

They are believed to have gained access to "top-secret exchanges with international partners".

A government spokesperson said: "We do not comment on individuals' security arrangements" but added that there are "robust systems in place to protect against cyber threats".

The hackers also gained access to Truss's conversations with her ally Kwasi Kwarteng criticising then-Prime Minister Boris Johnson, the report claimed.

Labour's Yvette Cooper, who focuses on homeland security, said the report raises "immensely important national security issues" including why and how the information was leaked.

"It is essential that all of these security issues are being investigated and addressed at the very highest level," she said.

Liberal Democrat foreign affairs spokesperson Layla Moran said: "We need an urgent independent investigation to uncover the truth."

The BBC and Sky News said they had not been able to verify the report.

A source told the paper the "compromised" phone has been placed inside a locked safe in a secure government location after up to a year's messages were hacked including "highly sensitive discussions" on the war in Ukraine.

The hacking was discovered in the summer when Truss was foreign minister and campaigning to become party leader and the next prime minister, the paper reported.

It claimed that "details were suppressed" by Johnson when prime minister and Simon Case, his most senior policy adviser.

The reported incident comes after interior minister Suella Braverman was reappointed by Prime Minister Rishi Sunak following her resignation over a security breach, in which she reportedly sent a top-secret document to an MP via her personal email.

The article did not make clear on what basis Russia was suspected to be behind the alleged attack.

But it quoted a security source as saying: "It takes a while to track who is behind attacks like these, but Russia tends to top the list."

Former Tory leader, MP Iain Duncan Smith told the newspaper: "Russia does this all the time."