Boris Johnson could lose his Uxbridge and South Ruislip seat despite the exit poll predicting a Conservative majority.
The Prime Minister is defending the smallest majority of any sitting Prime Minister since 1924.
Johnson won the seat by 5,034 votes in 2017, compared with 10,695 votes when he first one the seat in 2015.
The exit poll, published at 10pm today, predicted a Conservative majority of 86.
It forecast that the Conservatives would win 368 MPs. Labour are forecast to win 196 seats, the Liberal Democrats 13, the SNP 55, 22 for other parties and none for the Brexit Party.
The last sitting Prime Minister to lose their seat was Conservative Arthur Balfour at the 1906 election, where his party lost the election to the Liberal Party by a landslide.
The last Prime Minister to do the job while not a member of either House was Conservative Alec Douglas-Home, who served as PM from 1963-4.
He was a member of the House of Lords when he became PM.
He served as PM for 20 days between relinquishing his seat in the Lords and winning a Commons by-election.
There is nothing in the UK’s unwritten constitution to say that a sitting PM has to resign if they lose their seat.
It is likely that a Tory MP in a safe seat would stand down to make way for him in a by-election.
Robert Hazel, professor of politics at University College London, said: “I am sure there would be loyal Tory MPs willing to step aside if that enabled Boris Johnson to get swiftly re-elected to the Commons.
”He might have to offer some sort of inducement like a seat in the Lords.”
On the precedent set by Douglas-Home Douglas Hume he said it is ”harder to answer the question of how politically acceptable that might be nowadays.”