Andrew Bridgen, who like Mr Ross withdrew his letter of no confidence in Mr Johnson, said he was “disappointed that so many in Number 10, including the Prime Minister, were found to have breached rules they set”. He said now was “not the time to remove him given the international situation” but warned: “This is not the end of this matter.”

However, a former Cabinet minister said the biggest danger for Mr Johnson would be his previous Commons statements denying rule-breaking in Downing Street and “anything that appears to be a deliberate lie”.

The former minister said that while a 10-minute birthday party might let him off the hook, other more serious breaches at events he attended could emerge.

“He needs to go to Parliament and eat acres of humble pie and take it on the chin. He must not qualify it but be contrite,” he said. “It is not great, but we are not going to get rid of him for now. It still leaves a question mark over who leads us into the next election.”

Sir Keir Starmer, the Labour leader and a former director of public prosecutions, demanded the resignations of Mr Johnson and Mr Sunak and accused the Prime Minister of “repeatedly” lying about what happened at Number 10.

“The British public made the most unimaginable, heart-wrenching sacrifices, and many were overcome by guilt,” he said. “But the guilty men are the Prime Minister and the Chancellor. Britain deserves better – they have to go.”

Labour, the Liberal Democrats and SNP called for the Commons to be recalled from the two-week Easter break to allow Mr Johnson to “tender his resignation” to MPs in person. However, that is unlikely because it lies in the Government’s gift.

Sources close to Carrie Johnson, understood to have attended the birthday party “briefly” with her then new-born baby, confirmed that she had also paid her fine.

A spokesman said: “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.”