Matt Hancock's conduct may have threatened the security of private government communications, the Justice Secretary has admitted.
The former health secretary, who resigned over the weekend, is facing an investigation for allegedly using his private email account to conduct government business.
Asked whether Mr Hancock's actions were a "huge security issue" that could potentially see hackers gain access to government communications, Robert Buckland told the BBC:
"I agree, and that's why I think it's important that we use the systems that we are provided with."
Ministers are supposed to use their secure government emails and telephones for work to avoid hackers and foreign security services listening in on the inner workings of government.
The approach also ensures official correspondence is logged and subject to data protection and freedom of information laws.
Questions have been raised about whether Mr Hancock’s use of a personal email account will make it harder to scrutinise the process by which so many emergency Covid contracts came to be given to Conservative aquaintances and donors.
But commenting on the admission about security by Mr Buckland, Angela Rayner, Labour's deputy leader, said: “It’s staggering that a government minister has admitted that the Tory party could be putting national security at risk by carrying out government business on private emails but hasn't said that ministers are going to do anything about it.
"We already know that hostile actors target Ministers' private email accounts to access sensitive information.
"We need a full independent inquiry to get to the bottom of how wide this goes, whether Ministers have put our national security at risk and what steps will be taken to protect vital information and our country’s security.”
Speaking on Monday morning less than 48 hours after Mr Hancock's resignation, Mr Buckland claimed that under the rules, "anything that is very sensitive, I can assure viewers, isn't actually viewed on email".
On the question of an inquiry, he told Sky News: "We should use Government emails, I think that's very clear.
"I think the Cabinet Office, if they're asked to look at this, they probably will be, will need to satisfy themselves that if that was the case then the material is available."
Mr Hancock quit on Saturday following revelations about his relationship with an aide and CCTV footage of them embracing and apparently breaking social distancing instructions in a government office.
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