Sir Keir Starmer is facing calls to "change direction" from Labour’s left wing after he became the first leader to see the party lose Hartlepool in 47 years.
Labour suffered a "shattering" defeat in Thursday’s by-election as they lost out to the Conservatives by 6,940 votes.
The result boosts Prime Minister Boris Johnson’s majority in the House of Commons and also hands him another brick in Labour’s "red wall".
Conservative candidate Jill Mortimer, who will now become Hartlepool’s new MP, took more than half of all votes as the Tories leapfrogged Labour from second place in the constituency at the 2019 general election.
The Hartlepool vote – as well as this week’s local elections – were Sir Keir’s first major electoral test after just over a year as Labour leader.
And the by-election defeat focused scrutiny on his performance over the last 12 months, with critics from Labour’s left wing seizing on the result to urge Sir Keir to adapt his strategy.
Sir Keir was silent as he left his north London home on Friday morning.
But allies of Sir Keir’s predecessor, Jeremy Corbyn, were quick to point out Labour had twice won the Hartlepool seat under Mr Corbyn’s leadership in recent years.
"Not possible to blame Jeremy Corbyn for this result," Labour’s former shadow home secretary Diane Abbott posted on Twitter.
"Labour won the seat twice under his leadership. Keir Starmer must think again about his strategy."
Another of Mr Corbyn’s former shadow cabinet ministers, Richard Burgon, said Labour was "going backwards in areas we need to be winning".
"Labour’s leadership needs to urgently change direction," he added.
"It should start by championing the popular policies in our recent manifestos – backed by a large majority of voters."
And left-wing campaign group Momentum also accused Sir Keir of having taken Labour "backwards".
"It’s time to change direction, not double down on a failed strategy," they posted on Twitter.
"In order to rebuild, the leadership must build a coalition with the left on transformative policy, return to community organising, and empower members to shape the future of our party."
Mr Corbyn’s longest political ally, Labour’s former shadow chancellor John McDonnell, said Sir Keir had "to be given his chance".
But he claimed Labour had gone into the Hartlepool campaign "almost policy-less".
"We must never again send our candidates into an election campaign almost naked, without a policy programme, without a clear view on what sort of society you want to create," he told BBC Radio 4’s Today programme.
"That’s the sort of thing that we need now."
Labour former cabinet minister Lord Mandelson, a key architect of New Labour who was also MP for Hartlepool for 12 years, blamed the party’s defeat on longer-term problems.
He said he felt a "mild fury that the last 10 years of what we have been doing in the Labour Party nationally and locally has brought us to this result, because that is above all fundamentally an explanation of what’s happened today".
"What I would say is this, and remind the party we have not won a general election in 16 years," he told the BBC.
"We have lost the last four, with 2019 a catastrophe – the last 11 general elections read: lose, lose, lose, lose, Blair, Blair, Blair, lose, lose, lose, lose.
"We need for once in this party to learn the lessons of those victories as well as those defeats, and I hope very much that when Keir and his colleagues in the shadow cabinet say this means that we have got to change direction that they actually mean it."
Allies of Sir Keir pushed back at suggestions the party leader’s position could be imperilled by the Hartlepool result.
Shadow communities and local government secretary Steve Reed told Sky News the defeat was "shattering" but insisted there was no need to ditch Sir Keir.
"For the first time in many years, actually, we have a leader that people can see as an alternative prime minister," he said.
"What they don’t yet understand is, is Labour different from the Labour that they comprehensively rejected in December 2019? That job of work has not yet been done."
Mr Reed added the COVID pandemic had "in part" hindered Sir Keir’s efforts to establish himself with voters.
The party’s shadow schools minister, Wes Streeting, also backed Sir Keir. He tweeted: "Our leadership has changed for the better, but the voters aren’t convinced that Labour has too.
"This is a huge and urgent task. Keir gets it. So must we."