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After initially intending to stay in office, Mr Johnson has agreed to step down.
However, although the prime minister will resign, he will remain in office until the autumn as a caretaker prime minister.
A new Tory leader is set to be in place by the party conference in October, a No 10 source said.
On Wednesday, ahead of Mr Johnson’s announcement, the SNPs Westminster leader Ian Blackford called Johnson a “lame duck,” during the Prime Minister’s Questions session.
He said: "Week after week I’ve called on this prime minister to resign. I’ve been met with a wall of noise from the Tory benches. I thought they were trying to shout me down... when all this time it turns out that 41% of them have been cheering me on.
"Let’s be clear, at least the numbers don’t lie. 41% of his own MPs have no confidence in him. 66% of MPs across the House don’t support him, and 97% of Scottish MPs want the minister for the union shown the door.
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"We now have a lame duck prime minister presiding over a divided party in a disunited kingdom. How does the prime minister expect to continue when even unionist leaders in Scotland won’t back him?"
But what does a lame duck mean, and what powers will the prime minister have during his remaining time in office?
What is a lame duck?
A lame duck is “an ineffectual or unsuccessful person or thing.”
In politics, a lame duck refers to an outgoing political leader, whose successor has already been elected or will be shortly.
Due to their limited time left in office, a lame duck leader is seen as having less influence with the other politicians.
What powers will Boris Johnson have with his remaining time in office?
As he will officially remain prime minister until the autumn, Mr Johnson will theoretically have the same powers as he has always had.
But in reality, as an outgoing leader, he will not have the same authority or influence.
He will continue attending public appointments and representing the UK as the prime minister until a new leader of the Conservative party is elected.