There are mounting calls for Boris Johnson to resign amid the Downing Street Christmas parties scandal - but what actually happens if a prime minister resigns? 

Yesterday we saw the PM's former press officer Allegra Stratton step down after a video was leaked of her joking about the alleged party in a mock press conference on December 20 2020.

The video has caused outrage up and down the country, however Boris Johnson has continued to deny any rules were broken in Downing Street while the country was in lockdown last year.

Many political commentators have suggested that Stratton will not be the only one out of a job following this scandal, and the SNP as well as various media outlets have called for Boris Johnson's resignation.

But what happens if a prime minister resigns?

Here's what you need to know...

What happens if a prime minister resigns in the UK? 

It depends on the situation surrounding the resignation. 

If a prime minister resigns following the loss of a general election, they will offer their resignation to the monarch who will then invite the winning party to form a government in their name. 

However, if a prime minister resigns during a parliamentary session when their government is still in power and has not lost a general election, there is a different process. 

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. 

Have prime ministers resigned while parliament is sitting before? 

Yes, this has happened a few times. 

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.