The Tories romped to victory in the northern port down, which had been held by Labour since it was formed in 1974.
It has sparked a furious reaction from the Labour left, who are now calling for a change of direction, with many members demanding the current leader’s resignation.
Inevitably, the row was taken to social media, with the hashtag “StarmerOut” trending alongside a stream of brutal memes.
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Here are just some of the furious and mocking creations to which the Labour leader is now being subjected as he fights to keep his party together:
However, numerous Labour heavyweights have been quick to jump to Starmer’s defence.
Asked whether the party needs a new leader, former shadow chancellor John McDonnell said Starmer has “got to be given his chance”.
McDonnell told BBC Radio 4’s Today programme on Friday morning: “Keir’s got to be given his chance and I’ve said that all the way along.
“I’m not going to be one of those people treating (him) the way they treated Jeremy (Corbyn) – always challenging him, coups and all the rest.
“Keir now needs to sit down and think through what happened in this campaign, and what I’ve been saying to him is you need to demonstrate to people the sort of society you want to create, the policy programme that will achieve that society, and you need to get back to that real grassroots campaign.
“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.
“That’s the sort of thing that we need now.”
Meanwhile, Shadow communities secretary Steve Reed insisted the party would double down on the more centrist approach taken since Corbyn’s exit.
“What this shows is that, although we have started to change since the cataclysm of the last general election, that change has clearly not gone far enough in order to win back the trust of the voters,” he told BBC Breakfast.
The crushing Hartlepool result – which saw Tory candidate Jill Mortimer gain 15,529 votes compared to Labour’s Dr Paul Williams’s 8,589 – was due, in part, to voters who backed the Brexit Party in 2019 switching to the Conservatives.
But worryingly for Labour it saw its share of the vote fall, underlining the scale of the task facing Sir Keir if he is to chart a route to Number 10.
The party is now bracing for more difficult results as votes continue to be counted on Friday and over the weekend in the English local elections.
Reed denied Sir Keir’s leadership had been a problem and said they would work “tirelessly” to rebuild trust in the party.
“I’m very comfortable that we now have a leader that the country could see as an alternative prime minister – the problem is the Labour party itself,” he said.
“We have not yet changed the Labour Party enough for people to feel able to go out and trust it with their and their children’s futures.”