Labour has suffered a humiliating by-election defeat after losing Copeland to the Conservatives.
The Cumbrian seat has been held by the party since it was formed in 1983 but Tory Trudy Harrison snatched it by 2,147 votes in a historic victory.
It is the first time a governing party has taken a seat from the opposition for decades.
Harrison polled 13,748 votes to 11,601 for Labour’s Gillian Troughton.
The stunning Tory victory will heap pressure on Jeremy Corbyn over his divisive leadership of the party.
Labour’s majority in the Copeland at the general election was just 2,564.
But for an opposition to lose a seat to the party of power in a mid-term vote is extremely rare.
The last time it happened was the 1982 Merton, Mitcham and Morden by-election, although technically it was a Conservative gain from SDP as the sitting MP had defected from Labour to the SDP before the poll.
Before that, the closest comparable case was Sunderland South in 1953.
Labour earlier held Stoke-on-Trent Central after seeing off a concerted challenge from Ukip leader Paul Nuttall.
But Corbyn admitted the party had failed to get its message through in Cumbria.
He said: “Labour’s victory in Stoke is a decisive rejection of Ukip’s politics of division and dishonesty.
“But our message was not enough to win through in Copeland. In both campaigns, Labour listened to thousands of voters on the doorstep.
“Both constituencies, like so many in Britain, have been let down by the political establishment.
“To win power to rebuild and transform Britain, Labour will go further to reconnect with voters and break with the failed political consensus.”
The Conservatives increased their vote share by more than 8% in Copeland, while Labour’s was down by nearly 5%.
Harrison said her victory showed “the people are ready for change”.
She told Sky News: “I think we ran an extremely positive campaign and it was a campaign that represented the needs of this area.
“And I know this area because I have lived here all of my life.
“I think that, and the combination of Jeremy Corbyn’s views on nuclear in an area which is so dependent on Sellafield and on Moorside, contributed to my win tonight.”
In her victory speech, Harrison said the result was a “truly historic event”.
She said: “It’s been very clear talking to people throughout this campaign that Jeremy Corbyn doesn’t represent them.
“They want a party which is on the side of ordinary working people, which will respect the way we voted in the referendum and which will build a country which represents everyone. That’s why they voted for me tonight.”
John McDonnell, the shadow chancellor, said Corbyn’s position as leader was not under threat.
“It’s not a matter of hanging on. The situation is this: you learn lessons from these things and one of the lessons you learn is people will not vote for a divided party,” he told ITV.
“The last 18 months, 20 months, we’ve been involved in two leadership elections so understandably in a leadership election those divisions will come out.”
He added: “We can’t have a situation like we did last week where Tony Blair comes out and attacks his own party, Peter Mandelson as well.”