Jeremy Corbyn refused to consider his position after Labour suffered a historic by-election defeat in Copeland and narrowly held on to another seat in Stoke Central.
The Labour leader rejected calls to take responsibility for the defeat in Cumbria, where the Conservatives pulled off a shock victory by overturning a 2,564 Labour majority.
Trudy Harrison’s victory makes her the first Tory to represent the constituency since 1935. It is the first time a sitting government has won a by-election against the official opposition since 1982, and the scale of the win by a ruling party has not been seen since the Victorian era.
Labour breathed a sigh of relief as it held on to the other seat being contested on Thursday, with Gareth Snell holding on to Stoke Central on a reduced 2,620 majority.
There was a 2.14% swing against Labour, but the anticipated threat from UKIP, whose candidate was party leader Paul Nuttall, failed to materialised.
UKIP finished with 24.72% of the vote, just ahead of the Conservatives’ Jack Brereton on 24.35%.
If the results were repeated in an election, Theresa May could seize more than 50 Labour seats and expect a majority of well over 100 MPs.
Asked by reporters on Friday whether he accepted that Labour’s problems were his fault, Mr Corbyn said simply: “No.”
Earlier, the Labour leader had claimed that “both constituencies, like so many, have been let down by the political establishment”. In a speech on Labour’s response to Brexit, Mr Corbyn said he would oppose attempts to use the UK’s exit from the EU to slash workers’ rights and corporate taxes in a bid to reconnect with working class voters.
But he faced claims from his own party that voters had rejected his leadership, particularly in Copeland where workers at the Sellafield reprocessing plant did not welcome Mr Corbyn’s opposition to nuclear power. Labour figures also questioned the reliance on the issue of the NHS, with the campaign focusing on the downgrading of local maternity services.
John Woodcock, the Labour MP for neighbouring Barrow and Furness, said the result was “a disaster”.
“We should not attempt to insult anyone’s intelligence by claiming it is anything other than that,” he told the BBC.
Veteran Labour MP David Winnick called on Mr Corbyn to consider his position. “The party is faced with the problem of a leader who is simply not acceptable to a large number of people who would normally vote Labour,” Mr Winnick said. “That it is an obstacle and it would be wrong not to recognise that.”
Shadow Chancellor John McDonnell came to the defence of his leader, blaming the defeat on party disunity and accusing former grandees Tony Blair and Peter Mandelson of damaging Labour’s campaign by questioning Mr Corbyn’s stance on Brexit in the week before polling day.
Downing Street said the Prime Minister, who campaigned prominently in both constituencies, was told of the victory in Copeland by text in the middle of the night.
Despite the strong showing by the Tories, her spokesman ruled out an early election.
“The prime minister has set out the position very clearly on a general election,” he said. “There are no plans for one.”
Mr Nuttall, whose campaign in Stoke Central was dogged by allegation that he embellished his CV and inflated his involved in the Hillsborough disaster as 12 year-old child, also insisted he would stay on despite failing to become UKIP’s second MP.
He told journalists at the election count in the early hours of Friday that he was “not going anywhere”.
There were reports that former leader Nigel Farage had refused to campaign in the final stages of the election because of personal disputes with members of Mr Nuttall’s campaign team.