Labour leader Sir Keir Starmer and his deputy Angela Rayner have both received police questionnaires about the so-called “Beergate” event at a party office in Durham last year.
The party confirmed that both senior figures had been asked by Durham Constabulary to explain their attendance at the 30 April gathering, at which beer and curry was consumed.
The Durham force announced earlier this month that it would investigate potential breaches of Covid laws by Starmer and his staff, despite initially deciding against action when images of the meeting first appeared.
The Labour leader then announced a high-stakes gamble – that he would resign as leader if he is issued with a fixed penalty notice by the force.
“I believe in honour, integrity and the principle that those who make the rules must follow them,” said Sir Keir.
Ms Rayner, who was also at the Labour constituency office in Durham last spring, said she too would “do the decent thing and step down” if issued with a fixed penalty notice (FPN).
Both Sir Keir and Ms Rayner have continued to stress that they believe no Tier 2 rules in place at the time – which allowed for political campaigning – were broken.
Starmer said his team had “got on with their work” as the ate takeaway curry. “I simply had something to eat while working late in the evening, as any politician would do days before an election,” said the Labour leader.
Ms Rayner added: “Eating during a long day’s work was not against the rules.”
Senior Labour MP Nick Brown – chief whip under six party leaders – suggested earlier this week that Starmer should stand for re-election if he is fined and forced to resign.
“I don’t believe for one moment that the police will find against him. But let’s just for the sake of the argument, say they did,” the former Labour minister told the BBC’s Politics North.
Mr Brown added: “If they did, Keir would do what he said he would do and would resign. And I would be the first person urging him to seek re-election and to lead us into the next general election.”
It comes as Boris Johnson faces a steady flow of Tory MPs calling for his resignation following the damning report into Partygate by top civil servant Sue Gray.
John Stevenson, MP for Carlisle, became the latest Tory to call on Mr Johnson to resign over the Partygate “revelations” – telling colleagues to “facilitate a vote of confidence” in his leadership.
Under party rules Sir Graham Brady, the chairman of the backbench 1922 Committee, must call a vote of confidence if 54 Tory MPs submit a letter calling for one.
Close to 30 Tory MPs have publicly called on the PM to stand down. But more than 40 MPs have openly questioned his future, although not all of them have said whether they have written to Sir Graham.
Former Tory leader William Hague said Mr Johnson was now “in real trouble”, predicting that a vote on his leadership could come as early as next week.
