The hackers gained access to top private messages and communications with other nation's leaders, in addition to private messages between Truss and her close friend Kwasi Kwarteng, who later became finance minister, the report said.

Levelling-up secretary Michael Gove said that "he could not discuss any possible security breaches while insisting there were "robust protocols" in place over such issues, the Guardian reported. Home Scretary Suella Braverman's resigned after admiting "mistake" that breached government security rules, which Rishi Sunak later restored as Home Secretary. 

"The messages are believed to have included discussions with senior international foreign ministers about the war in Ukraine, including details about arms shipments, Daily Mail wrote. Up to a year's worth of messages were downloaded, the Mail said, citing unnamed sources.

"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," quoting a British government spokesperson Reuters wrote.

The hack was uncovered during the Conservative Party leadership campaign that led to Truss becoming prime minister, the Mail reported.

Truss left office last week as prime minister and was succeeded by Rishi Sunak.