British Labour leader Keir Starmer has said he will do "whatever is necessary" to rebuild trust in Labour following its "bitterly disappointing" defeat in the Hartlepool by-election.
The Conservative Party scored an upset victory in the Labour stronghold, after Britain held its first major electoral test since Brexit and the coronavirus crisis.
In a stunning result, the Conservative candidate Jill Mortimer overturned a majority of 3,500 at the general election to take the seat - which had been Labour-held since it was formed in 1974 - with a majority of 6,940 over Labour's Dr Paul Williams.
After seeing another pillar in its once impregnable "red wall" fall, Mr Starmer told his party to "stop quarrelling among ourselves" and address the needs of the country.
The bruising result - described as "absolutely shattering" by one shadow cabinet minister - prompted calls from across the party for a change of direction.
With Labour braced for further damaging losses in English council elections, Mr Starmer said he was determined to address the problems.
"I'm bitterly disappointed in the result and I take full responsibility for the results - and I will take full responsibility for fixing this," he said.
"We have changed as a party but we haven't set out a strong enough case to the country.
"Very often we have been talking to ourselves instead of to the country and we have lost the trust of working people, particularly in places like Hartlepool.
"I intend to do whatever is necessary to fix that."
Meanwhile British Prime Minister Boris Johnson has said the UK was looking to the government to deliver on the public's priorities.
Speaking in Hartlepool, Mr Johnson said: "What has happened is that they can see we did get Brexit done and, to a certain extent, they can see that we delivered on that.
"What people want us to do now is to get on with delivering on everything else.
"Number one is continuing the vaccine roll-out, making sure that we go from jab to jab, to jab to jobs, jobs, jobs."
Mr Johnson said the UK government has embarked on a "massive project" of "uniting and levelling up" the country.
"Every government has tried it to some extent but I don't think any government has tried it as wholeheartedly as this government is trying," he said.
The winning candidate Jill Mortimer declared the win a "truly historic result".
The defeat was signalled by senior figures hours before the official announcement, with Labour's transport spokesperson Jim McMahon telling Sky News shortly before 3am his party was "not close to winning this".
The result will be a setback for Keir Starmer, who took over from Jeremy Corbyn four months after the party's disastrous 2019 general election performance with the promise of turning it back into a winning force.
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Momentum, the left-wing Labour group set up during Mr Corbyn's leadership, tweeted after the Hartlepool defeat: "This result is a disaster.
"In 2017, we won over 50% of the vote in Hartlepool. Now it looks like we've lost it to the Tories.
"A transformative socialist message has won in Hartlepool before, and it would have won again."
The result is likely to mean questions over the strategy Mr Starmer has pushed as leader over the past year, with traditional Labour voters seemingly continuing to turn away from the party in the wake of Brexit.
Early results in council contests after the Super Thursday elections appeared to show voters deserting Labour, with the Tories seizing Redditch and Nuneaton & Bedworth councils in the Midlands, along with Harlow in Essex, while Labour saw heavy losses across North East local authorities.
A Labour source said Mr Starmer would "take responsibility for these results" and for "fixing" the party's electoral woes.
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.
Both Mr Johnson and Mr Starmer made three visits during the campaign in a sign of the importance the by-election represented to their parties.
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 of the elections - which also include the Welsh Parliament, police and crime commissioners and English local authorities and mayors - are expected to continue filtering through until Monday as counting will take longer than normal due to coronavirus restrictions.
In Wales, Mark Drakeford hopes to maintain Labour's grip on the Senedd - but he may find himself forced to forge a new coalition to stay as First Minister.
That could mean talks with Plaid Cymru, whose leader Adam Price has committed to an independence referendum within five years if his party wins a majority.
