It follows a string of protests against his leadership. 

How long will Boris Johnson remain Prime Minister?

Boris Johnson has spoken to Tory 1922 Committee chairman Sir Graham Brady and agreed to stand down, with a new Tory leader set to be in place by the party conference in October, a No 10 source said.

What happens now that Boris Johnson has resigned?

But as Boris Johnson leaves No 10, what happens if a Prime Minister resigns?

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.