A Labour source said Keir Starmer would “take responsibility” for the result as the party’s left-wing demanded the leader “urgently change direction”.
The seat has been Labour since its creation in 1974. But Conservative Jill Mortimer has been elected Hartlepool’s new MP with a majority of 6,940, a 16% swing from Labour to the Tories.
It is highly unusual for a governing party to win a by-election.
Hartlepool was held by Labour with a majority of 3,595 in 2019, even as other bricks in the red wall crumbled – in part due to the Brexit Party splitting the Tory vote.
Before the count was finalised, Labour all but conceded defeat with shadow cabinet member Jim McMahon admitted the party was “not close to winning this”.
The victory cements the Tory party’s growing popularity in the north of England following the 2019 general election, where a string of seats in Labour’s traditional heartlands fell to Boris Johnson’s party.
Early results in council contests elsewhere also appeared to show voters deserting Labour, as ballots continue to be tallied up across England, Scotland and Wales following the “Super Thursday” polls – the largest test of political opinion outside a general election.
The Tories seized Redditch and Nuneaton and Bedworth councils in the Midlands from Labour, along with Harlow in Essex, while Starmer’s party saw heavy losses across North East local authorities.
A Labour source said Starmer would “take responsibility for these results” as he sought to “win back the trust and faith of working people”.
“The message from voters is clear and we have heard it – Labour has not yet changed nearly enough for voters to place their trust in us,” they said.
“We understand that. We are listening. And we will now redouble our efforts.”
In a sign of the discontent on the Labour left, MP Lloyd Russell-Moyle appeared to mock the party’s attempts to change its image. He said: “Good to see valueless flag waving and suit wearing working so well … or not?”
Richard Burgon, a shadow cabinet minister during Jeremy Corbyn’s time as leader, said the result in Hartlepool showed the party was “going backwards in areas we need to be winning”.
“Labour’s leadership needs to urgently change direction. It should start by championing the popular policies in our recent manifestos - backed by a large majority of voters.”
Shadow cabinet minister Thangam Debbonaire admitted the party’s new messaging was “not cutting through”.
Following the Hartlepool declaration, attention will turn to results elsewhere as ballots continue to be counted across England, Scotland and Wales in the largest test of political opinion outside a general election.
Results from the Holyrood election – where the issue of Scottish independence was a main feature in the campaign – will come through later on Friday and Saturday.
This article originally appeared on HuffPost UK and has been updated.