The claim that former health secretary Matt Hancock used a private email account to conduct government business should be fully investigated, the Labour Party has said.
Deputy leader Angela Rayner called for a probe into whether ministers were "using private emails to conduct official government business in secret".
Hancock had been under pressure to quit after The Sun newspaper published photos and then video of him and aide Gina Coladangelo, who are both married with three children, kissing inside the Department of Health and Social Care on 6 May.
The Sunday Times reported that minutes of meetings seen by the newspaper revealed that the former health secretary had been using a private Gmail account since March 2020, which meant that key decisions and their reasoning were not recorded or could be difficult to access for any future inquiry into the handling of the coronavirus pandemic.
Watch: Justice secretary says no 'dilly-dallying' over Matt Hancock departure
The Department of Health and Social Care insisted all ministers conduct their government business through departmental email addresses.
On Monday, Angela Rayner said there must be “full transparency” and a “full investigation”.
She told BBC Radio 4's Today programme: “I’ve written to ensure that there is an investigation into ministers using private emails to conduct official government business in secret, agreeing contracts in private etc.
“We need full transparency on this and a full investigation.”
Justice secretary Robert Buckland said ministers must use government emails.
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.”
Cabinet Office guidelines say that if personal accounts are used for government business, either the sender or receiver must “take steps to ensure the relevant information is accessible (e.g. by copying it to a government email address)” for record-keeping purposes but also so it can be requested under the Freedom of Information Act.
There may also be security risks if sensitive information is sent via private email.
Buckland said Hancock “frankly worked without a break” to tackle the coronavirus pandemic as health secretary.
He told the Today programme on Monday: “Matt Hancock is no longer the health secretary – he resigned on Saturday, and I’m amazed we’re having a discussion like this.
“He resigned because he considered the matter carefully, he could see the issue of credibility was one that was really majoring.
“Matt Hancock had been I think an incredibly hard-working health secretary over the last three years, in fact, but in the last 16 months somebody who frankly worked without a break to deal with this crisis.”
Hancock announced his resignation in a Twitter video on Saturday, saying: "I understand the enormous sacrifices that everybody in this country has made, that you have made, and those of us who make these rules have got to stick by them and that's why I have got to resign."
Labour says questions remained over a number of issues, including how Coladangelo – a friend of Hancock from university – was hired as an unpaid adviser, and then to a £15,000 role at the Department of Health and Social Care.
It was reported that she will also leave her role at the department.
Labour leader Sir Keir Starmer said: “If anybody thinks that the resignation of Matt Hancock is the end of the issue, I think they’re wrong and I think the incoming health secretary and the prime minister now have serious questions to answer about the CCTV, about the access, the passes, the contracts.”
The government said it will launch an internal investigation into how CCTV footage of Hancock was leaked.
Hancock's replacement as health secretary is Sajid Javid, a former chancellor and home secretary. He is set to make his first Commons statement on Monday since returning to the Cabinet.
He said on Sunday his “most immediate priority” would be “to see that we can return to normal as soon and as quickly as possible”.
While it is not expected he will bring what is left of England's coronavirus lockdown to a close any earlier than 19 July, he is said to be confident the measures will not extend past that date.
Watch: New health secretary Sajid Javid sets off to work