Labour have lost the Copeland by-election, a seat they had held for over 30 years, as the Conservatives snatched victory by more than 2,000 votes.
Trudy Harrison polled 13,748 votes while Labour's Gillian Troughton recorded 11,601, meaning they have lost a seat held by them since 1983.
The humiliating defeat piles more pressure on party leader Jeremy Corbyn, who experienced mixed fortunes during Thursday's two by-elections.
Just 30 minutes before the result was announced, Labour learned they had held onto the Stoke-on-Trent Central stronghold by 2,000 votes, brushing aside Ukip leader Paul Nuttall in the process.
But the Conservative's victory in Copeland, where turnout was 51.35%, is the first time a governing party has taken a seat from the opposition for decades.
Labour's majority in the constituency 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.
While hailing victory in Stoke, Mr Corbyn admitted that Labour's message was "not enough to win through" in Copeland.
After her victory, Ms 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."
Mr Corbyn 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."
Full election result breakdown:
Trudy Harrison (C) 13,748
Gillian Troughton (Lab) 11,601
Rebecca Hanson (LD) 2,252
Fiona Mills (Ukip) 2,025
Michael Guest (Ind) 811
Jack Lenox (Green) 515
Roy Ivinson (Ind) 116