Boris Johnson's aides reportedly banned him from taking top secret documents home after confidential papers were left strewn around his flat.
Highly classified material, known as Strap documents, was allegedly found lying around in the Prime Minister's Downing Street flat and in the upstairs quarters at his Chequers retreat.
The PM's former top aide Dominic Cummings ordered a crackdown on intelligence documents being put in Mr Johnson's red box in early 2020 after becoming alarmed by the "frat house" atmosphere, according to the Sunday Times.
It comes as separate claims emerged that Whitehall enforcer Sue Gray has been informed that some friends of the Prime Minister's wife Carrie had the access code to the Downing Street flat.
Downing Street did not comment on the allegations.
The latest revelations add to a growing picture of behind-the-scenes chaos as Mr Johnson battles to save his premiership, which has been rocked by the partygate saga.
The Strap papers, which are distinctive because of their pink colour, were reportedly in view in the flat while guests were there.
Martin Reynolds, the PM's principal private secretary, is said to have insisted Mr Johnson approved sensitive documents in his No10 office.
“[Dominic] Cummings spoke to the PPS and they agreed on new protocols to stop sensitive Strap material going to the flat or upstairs at Chequers,” a No 10 source said.
“Instead material was to be shown to the PM downstairs in No 10 or Chequers, then immediately returned to safekeeping.”
Another source told the paper that the PM's ministerial box was also left outside the door of the flat on Saturdays.
“It would be there in the morning and often still there in the evening,” the official said. “He wouldn’t have touched it.”
Nick Timothy, Theresa May's former joint chief of staff, tweeted: "For those making light of STRAP material, it's classified that way because it often depends on covert human intelligence sources.
"Which means if you're indiscreet with the information, you're putting the source's life in danger."
Lib Dem business spokesperson Sarah Olney told the Mirror: "In serious times we need a serious Prime Minister who is up to the task, but it is clear Boris Johnson can't even control what happens under his own roof.
"With Russian troops mounting on the Ukraine border and families drowning in energy bills, Boris Johnson needs to get a grip.
"The British public has lost faith in Boris Johnson. People see he is not up to the job and stories like this only dent public confidence in him further."
Mr Johnson has been fighting to win over mutinous MPs who were openly plotting his downfall in recent weeks.
Today, Foreign Secretary Liz Truss insisted Mr Johnson was "absolutely" the best person to lead the Conservative Party into the next general election.
Asked if she is tempted by the top job, she told the BBC's Sunday Morning programme: "There is no contest. There is no discussion."
On whether Mr Johnson is the best leader to take the Tories into the next general election, she said: "Absolutely he is."
She added: "He has achieved an 80-seat majority for the Conservative Party. He has delivered on the Covid vaccine programme, the booster programme, he's delivered on Brexit and he will do a fantastic job winning the next election for the Conservative Party."
Sue Gray's report into the Downing Street party saga is expected to be handed to the PM in the coming days.
But the Metropolitan Police's demand that she make "minimal reference" to parties it is investigating has sparked fears it could be watered down.
Ms Truss admitted "your guess is as good as mine" when asked when the report could come, but she insisted it was still the Government's intention to publish the document in full.
Meanwhile, it emerged that Sue Gray had been informed about an alleged party in the PM's flat the night Dominic Cummings resigned.
The Mirror reported last year that No10 aides “drank their socks off" at the gathering held during the November 2020 lockdown, with staff reporting hearing "loud music pumping out of the flat" late into the night
Ms Gray has been told about the gathering, where aides blasted Abba's 'The Winner Takes it All' after Mrs Johnson's rival was ousted, according to the Mail on Sunday.
A spokeswoman for Mrs Johnson dismissed the claims at the time as "total nonsense”, adding: "Mrs Johnson has followed coronavirus rules at all times and it is categorically untrue to suggest otherwise.”
