The British government insists it has robust cyber security for government officials, after a newspaper reported former prime minister Liz Truss’s phone was hacked while she was UK foreign minister.

The Mail on Sunday said the hack was discovered when Ms Truss was running to become Conservative Party leader and prime minister in the summer.

It said the security breach was kept secret by then-prime minister Boris Johnson and the head of the civil service.

The newspaper, citing unnamed sources, said Russian spies were suspected of the hack.

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The UK government spokesperson declined to comment on security arrangements, but said it had “robust systems in place to protect against cyber threats”, including regular security briefings for ministers.

Opposition parties demanded an independent investigation into the hack, and into the leak of the information to a newspaper.

“Was Liz Truss’s phone hacked by Russia, was there a news blackout and if so why?” asked Liberal Democrat foreign affairs spokesperson Layla Moran.

“If it turns out this information was withheld from the public to protect Liz Truss’s leadership bid, that would be unforgivable.”

Labour Party law-and-order spokesperson Yvette Cooper said “the story raises issues around cyber security”.

“It’s why cyber security has to be taken so seriously by everyone across government, the role of hostile states,” she told Sky News.

“But also the allegations about whether a cabinet minister has been using a personal phone for serious government business, and serious questions about why this information or this story has been leaked or briefed right now.”