Boris Johnson is once again facing allegations of  breaking coronavirus rules after Downing Street admitted he had a birthday celebration inside No 10 during the first lockdown.

Downing Street conceded staff “gathered briefly” in the Cabinet Room following a meeting after it was alleged 30 people attended and shared cake despite social mixing indoors being banned.

ITV News reported the Prime Minister’s wife, Carrie Johnson, had organised the surprise get-together complete with a chorus of “happy birthday” on the afternoon of June 19 2020.

Interior designer Lulu Lytle admitted attending but insisted she was only present “briefly” while waiting to talk to Mr Johnson about the lavish refurbishments she was carrying out to the couple’s flat above No 11.

A Downing Street spokeswoman said: “A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than 10 minutes.”

The Prime Minister has faced waves of public anger with each revelation and has faced calles to resign from within his own party.

But as Boris Johnson faces fresh scrutiny, what happens if the Prime Minister was to resign?

Here is all you need to know.

What happens if a Prime Minister resigns in the UK?

This will depend on the situation.

If a Prime Minister loses a general election they will offer their resignation to the monarch who will then invite the winning party to form a new government in their name.

However, if a Prime Minister resigns while their government is still in power, the process differs.

In this case, the government is still elected and therefore the resignation of the standing prime minister triggers a leadership contest, where fellow members of the government will campaign to be leader of the party.

When the leadership contest is decided, the outgoing prime minister will go and offer their resignation to the Queen, and advise she appoints the new leader of the party as prime minister. 

This new prime minister is not then required to hold a general election, but they often choose to soon after they take over. 

Has a UK Prime Minister resigned before?

Yes, it happens quite regularly.

When Tony Blair announced his resignation on May 10 2007, he triggered a Labour leadership contest. 

Gordon Brown received 313 nominations for leader, and his only opponent, John McDonnell, soon conceded defeat. 

On June 27 2007, Brown officially became prime minister of the UK. 

The same thing happened with David Cameron's resignation on June 24 following the Brexit vote.

Theresa May won the Conservative leadership contest after her being left the sole candidate. 

When May then announced she would resign on June 7 2019, another leadership contest was triggered, resulting in the election of Boris Johnson over Jeremy Hunt.