Russian spies were suspected of the hack, the newspaper said, citing unnamed sources.
A UK government spokesman declined to comment on security measures, but said he had “robust systems in place to protect against cyber threats”, including regular security briefings for ministers.
Opposition parties have called for an independent investigation into the hacking and leaking of information to a newspaper.
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“Was Liz Truss’s phone hacked by Russia, was there a news blackout, and if so, why?” said Liberal Democrat foreign affairs spokeswoman Leila Moran. “If it turns out that this information was withheld from the public to protect Liz Truss’s leadership candidacy, it will be inexcusable.”
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“The story raises issues related to cybersecurity,” Labor Party law and order spokeswoman Yvette Cooper said.
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“That’s why everyone in government needs to take cybersecurity, the role of hostile states, so seriously,”
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she told Sky News. “But also, accusations about whether a cabinet minister is using a personal phone for serious government business, and serious questions about why this information or this story has just been leaked or reported.”