Jeremy Corbyn has said he will not lead Labour into the next election, following a "very disappointing night".
With one seat left to declare, the party has won 203 seats.
Mr Corbyn said he would stay on as leader during a "process of reflection", and said Brexit had "polarised" politics which had "overridden normal political debate".
But others within Labour, including former MPs who lost their seats, blamed Mr Corbyn's leadership.
BBC political editor Laura Kuenssberg said Mr Corbyn was not intending to resign and it could take until April for a leadership contest to take place.
On the night, the Conservatives won a big majority, sweeping aside Labour strongholds across northern England, the Midlands and Wales in areas which backed Brexit in the 2016 referendum.
Some traditional Labour constituencies, such as Darlington, Sedgefield and Workington, in the north of England, will have a Conservative MP for the first time in decades - or in the case of Bishop Auckland and Blyth Valley - for the first time since the seat was created.
At 33%, Labour's share of the vote is down around eight points on the 2017 general election and is lower than that achieved by Neil Kinnock in 1992.
Speaking at his election count in Islington North, where he was re-elected with a reduced majority, Mr Corbyn said Labour had put forward a "manifesto of hope" and criticised the "way the media behaved" towards his party during the campaign.
But he added: "Brexit has so polarised and divided debate in this country, it has overridden so much of a normal political debate."
"I recognise that has contributed to the results that the Labour Party has received this evening all across this country."
Labour primarily campaigned on a promise to end austerity by increasing spending on public services.
The party also promised to renegotiate Prime Minister Boris Johnson's Brexit deal, and then put it to a referendum vote alongside the option of remaining in the EU.
That strategy was criticised by party chairman Ian Lavery, who said it had led voters in traditional Labour seats to believe it was "a Remain party".
"They believe they should have been listened to - and they think that the Labour party have totally reneged on the result," he said.
But he added the strategy was not "Jeremy Corbyn's decision", as it had been approved by delegates at the party's September conference.
Former Labour MP John Mann said the leader's unpopularity on the doorstep was palpable and Mr Corbyn should have "gone already".
Others have blamed the party's support for another Brexit referendum and the long-running anti-Semitism row.
Margaret Hodge, MP for Barking, said Labour had become the "nasty party".
Given the result, you might assume Jeremy Corbyn would swiftly fall on his sword - but he has instead called for a period of quiet reflection.
Party rules make it difficult to oust him, but already senior figures are asking how long this period will last.
Senior figures at Westminster and in local government feel delaying an inevitable leadership contest will lead to a similar result in May's council elections.
Mr Corbyn seems intent on staying in place until someone from his wing of the party is ready to take over - but the defeat of shadow minister Laura Pidcock has eliminated one of the potential left-wing leadership challengers.
Those who would prefer shadow Brexit secretary Sir Keir Starmer are keen that a new leader is in place soon to challenge Prime Minister Boris Johnson's Brexit policy.
The battle to establish the reasons for the defeat has already begun.
The narrative from the leadership that Brexit was to blame will be challenged robustly by those who want the party to change direction.
Shadow chancellor John McDonnell, a key ally of Mr Corbyn, said he was "heartbroken" at the result and insisted he would not take on the party leadership temporarily.
"At some stage we'll go into a leadership election," he said.
"Jeremy wants to ensure there is a period of reflection."
Earlier, he said he did not think the Labour leader had been "the big issue" of the campaign.
But former Labour justice secretary Lord Falconer called for the party to move quickly to replace Mr Corbyn as leader by March or April.
Gareth Snell, who lost his Leave-backing Stoke-on-Trent Central seat, called for both Mr Corbyn and Mr McDonnell to quit.
He accused senior figures in the shadow cabinet, who are defending Remain-voting seats in London, of "sacrificing" candidates in marginal constituencies in the Midlands and the north of England.
Elsewhere in the city, Ruth Smeeth, who lost her Stoke-on-Trent North seat to the Conservatives, described the election result as "devastating".
"For me, this is about whether the Labour Party has any right to exist [and] whether we have anything left to say," she said.
Another Labour MP to lose her seat, Caroline Flint in Don Valley, said: "So many of my voters could not and did not want to support Jeremy Corbyn to be prime minister."
She added: "There are moderate MPs who have driven us into a dead-end regarding Brexit and they have put the pursuit of Remain at the expense of our working-class heartlands and I feel annoyed, to say the least, about that."
Shadow Brexit secretary Sir Keir Starmer, speaking after holding his Holborn and St Pancras seat, said: "As a whole movement, we need to reflect on this result and understand it together, but we also have a duty to rebuild, starting now."
Yvette Cooper, who unsuccessfully challenged Mr Corbyn for her party's leadership in 2015, said the results showed Labour has "to change as a party".
She said Brexit had played a "significant part" in her party's performance, but the election "was not just about Brexit".
"It was about their perceptions of the party, their perceptions of the leadership," she added.
Speaking after an earlier exit poll predicted heavy losses for Labour, former Labour home secretary Alan Johnson told ITV News that Mr Corbyn had been "incapable of leading" and "worse than useless at all the qualities you need to lead a political party."
Polling expert Professor Sir John Curtice said: "It was clear that lots of Remain voters had doubts about Jeremy Corbyn's ability to handle Brexit and indeed, more broadly to handle anything."
In a statement, the Jewish Labour Movement - affiliated to the party for a century and representing about 2,500 members - said Labour's failure "lies squarely with the party's leadership".
"Because of the public's rejection of Corbyn as prime minister, the confused position on Brexit, or its total failure to tackle anti-Jewish racism, the party must truly listen," the statement said.
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