Boris Johnson is facing increasing pressure from both sides of the aisle to step down over the ongoing Downing Street party scandal, being referred to as Partygate.
Mr Johnson insisted today that "nobody told me " that the boozy knees-up he has admitted attending broke the rules in place at the time.
Now No 10 has said that the Prime Minister will step down if he misled Parliament, but only if he did so knowingly.
A spokesperson for the PM has said Mr Johnson "abides" by the ministerial code, whereby ministers who have "knowingly mislead Parliament will be expected to offer their resignation".
Deputy Prime Minister Dominic Raab has said that "normally" a PM would be expected to resign, although he insisted the idea was "hypothetical".
The deputy had been sent out to come to the PM's aid following attacks from his former aide, Dominic Cummings, who claimed Mr Johnson lied to Parliament over Partygate.
Who is the deputy Prime Minister?
At the time of writing, the deputy Prime Minister is a position held by former Foreign Secretary Dominic Raab.
Mr Raab drew fury last year when, during the exodus from Afghanistan following the US military withdrawal, Mr Raab was slow to return to the UK from his holiday in Crete.
He said "with hindsight" he would not have gone away at all, although he was "caught unawares" by the speed with which the Taliban retook the country.
Shortly after the crisis, Mr Raab was removed as Foreign Secretary during a Cabinet reshuffle by the PM. During the reshuffle, he was appointed as the Justice Secretary and Deputy Prime Minister.
Mr Raab is the first person to officially occupy the role since Liberal Democrat Nick Clegg, who held it as part of the coalition government between the Tories and the Lib Dems formed in 2010.
The government at the time was often referred to as the Cameron-Clegg coalition.
The role of deputy has also been held in the past by political big hitter John Prescott during the Blair years and Michael Heseltine during the John Major premiership.
Would the Deputy Prime minister take over if Boris steps down?
The exact procedure of what happens if a PM resigns can vary depending on the context. The deputy has no automatic right to the top job despite technically being second in command.
If a Prime Minister resigns after they have lost a general election, they hand in their letter to the Queen and the winning party takes over.
However, if someone resigns during parliament, a leadership election would be triggered immediately. This is because it is in the UK's unwritten constitution that the country must have a PM at all times.
This process could take a month, but when it is decided the outgoing PM will give their resignation to the Queen and ask her to invite their successor to form a government.
This is what happened when David Cameron resigned and Theresa May took over.
If a Prime Minister were to die in the job, the role would usually be taken up in the short term by the Deputy Prime Minister and the same applies for illness.
When Mr Johnson went into intensive care with severe Covid symptoms in April 2020, he asked Mr Raab to deputise for him.
The office of deputy isn't always used, with Mr Raab being the first person to fill the position officially since 2015.
It is the Deputy Prime Minister's role to stand in for the PM if they cannot be present for the weekly Prime Minister Questions, traditionally held on Wednesday at midday when parliament is in session.
They would also chair Cabinet meetings if the PM is unable to attend.
