Boris Johnson will be fined for attending his own birthday bash in the Cabinet Room of No 10 during Covid lockdown.

No 10 said the Metropolitan Police had provided more clarity about why the Prime Minister had been told he would receive a fixed-penalty notice (FPN) as part of the probe into alleged lockdown parties at the heart of Government.

Chancellor Rishi Sunak and the Prime Minister’s wife, Carrie Johnson, are among those told to expect a fine, with Mrs Johnson’s spokeswoman confirming that her FPN also relates to her husband’s birthday celebration.

A spokesman for No 10 said: “The Met Police have now explained that the FPN issued to the PM will be in relation to the following incident: “On June 19 2020 at the Cabinet Room, 10 Downing Street, between 1400 and 1500 you participated in a gathering of two or more people indoors.”

The birthday party was reportedly a surprise do for the Prime Minister, organised by his wife and for which the Chancellor was also in the room.

Northern Ireland minister Conor Burns previously said Mr Johnson was “ambushed by a cake” at the event, at which up to 30 people are said to have gathered and sung Happy Birthday.

A spokesperson for Mrs Johnson said the former Tory director of communications had paid the fine, which is understood to have been £50 if settled within 14 days,

“Whilst she believed that she was acting in accordance with the rules at the time, Mrs Johnson accepts the Metropolitan Police’s findings and apologises unreservedly,” her spokeswoman said.

PA news agency has been told that Mrs Johnson “briefly attended” the Cabinet Room with their then-newborn baby, Wilfred, during the PM’s lunch break and alongside “some socially distanced staff”.

Labour leader Sir Keir Starmer has called for the Prime Minister and Mr Sunak to resign following the issuing of fines for breaking coronavirus laws.

Speaking in Preston, the former director of public prosecutions said the police’s decision marked the “first time in the history of our country that a prime minister has been found to be in breach of the law”.

He accused Mr Johnson of “repeatedly” lying about what happened behind the famous black door of No 10 and said he and Mr Sunak had “dishonoured” the sacrifices made by Britons during the coronavirus pandemic.

“Britain deserves better, they have to go,” Sir Keir added.

His party has joined the chorus of demands for the Commons to be recalled from its two-week Easter break and allow Mr Johnson to “tender his resignation” in person to MPs.

The push for a recall has also been made by the Liberal Democrats and Plaid Cymru, while Scottish First Minister Nicola Sturgeon has been among those of prominence to demand the PM quits.

A Labour spokeswoman said: “Boris Johnson promised he would give a statement to the House of Commons, so we want to hear from the Prime Minister at the earliest possible opportunity to give him the chance to correct his lies and tender his resignation.

“Obviously it is only the Government who actually has the power to ask the speaker to recall Parliament.”

Scotland Yard said on Tuesday that it had made at least 50 referrals for fixed-penalty notices – up from 20 at the end of March – to ACRO Criminal Records Office, which is responsible for issuing the fines, as part of Operation Hillman.

Mr Johnson is understood to have been present at six of at least 12 events being investigated, with the Met Police not ruling out issuing further punishments as the inquiry continues.

The Met has opted not to name individuals who have been fined as part of its probe into lockdown breaches in No 10 and Whitehall.

But, following the announcement about another 30-plus fines being referred, No 10 confirmed that Mr Johnson and Mr Sunak were among those to be included in the latest tranche of FPNs.

Amanda Milling, a Foreign Office minister and former Tory Party chair, led the defence of the Prime Minister as the first minister to speak out in support.

Culture Secretary Nadine Dorries followed on, arguing that the Tory leader had “been clear about what happened on” in June 2020 and had “offered a full apology”.

“It was a brief gathering in the Cabinet Room, less than 10 minutes during a busy working day,” said Ms Dorries.

Veteran Tory MP Sir Roger Gale, a vocal critic of the Prime Minister, said now was not the time to “unseat” Mr Johnson, with Russia’s attack on Ukraine to deal with.

The North Thanet MP told the PA news agency there was an “international crisis” and that nothing should be done to “destabilise the coalition” against Russian president Vladimir Putin.

Sir Roger said the Prime Minister would need to acknowledge that he was “patently wrong” to have argued that no rules were broken in Government.

Scottish Tory leader Douglas Ross echoed his colleagues’ sentiments, saying it “wouldn’t be right” to remove the Prime Minister during the Russian invasion of Ukraine.

Mr Ross had previously submitted a letter of no confidence in Mr Johnson’s leadership – a method used for toppling a Tory leader if there is enough dissenters – but withdrew it following the outbreak of war in eastern Europe.

It comes as a poll found that a majority of people in Britain think the Prime Minister should resign after being issued with a Covid fine.

A snap YouGov poll of 2,460 adults taken on Tuesday found that 57% of people think he should go, with the same proportion thinking the same about the Chancellor.