The breach was discovered when Truss, then the foreign secretary, was running for the Tory leadership in the summer, but details were suppressed by the then-prime minister, Boris Johnson, and the cabinet secretary, Simon Case, the Mail reported on Sunday.
Spies suspected of working for Russia gained access to sensitive information, including discussions about the Ukraine war with foreign officials, the newspaper said, citing unnamed sources, The Guardian reported.
It also claimed private conversations between Truss and Kwasi Kwarteng criticizing Johnson fell into the hackers’ hands, leaving them at risk of blackmail.
Opposition parties are demanding an investigation into the alleged attack, which will raise questions about the UK’s cybersecurity, as well as the judgment of Johnson and Case.
Labor’s shadow home secretary, Yvette Cooper, said: “There are immensely important national security issues raised by an attack like this by a hostile state which will have been taken extremely seriously by our intelligence and security agencies.
“There are also serious security questions around why and how this information has been leaked or released right now which must also be urgently investigated.
“It is essential that all of these security issues are investigated and addressed at the very highest level and we need to know that the government recognizes the gravity of this and the importance of fully protecting our national security.”
The Liberal Democrat foreign affairs spokesperson, Layla Moran, said: “We need an urgent independent investigation to uncover the truth. Was Liz Truss’s phone hacked by Russia, was there a news blackout and if so why?
“If it turns out this information was withheld from the public to protect Liz Truss’s leadership bid, that would be unforgivable.”
A government spokesperson said: “We do not comment on individuals’ security arrangements.
“The government has robust systems in place to protect against cyber threats. That includes regular security briefings for ministers, and advice on protecting their personal data and mitigating cyber threats.”