British Conservatives won a Westminster seat in the Labour stronghold of Hartlepool, strengthening Prime Minister Boris Johnson's grip on power, official results showed on Friday.
Conservative candidate Jill Mortimer won 15,529 votes, almost double that of her Labour opponent Paul Williams who obtained 8,589 votes.
The by-election in the north-eastern constituency took place following the resignation of the previous MP Mike Hill.
The seat had been held by Labour MPs for almost 60 years.
The Tories' victory deals a harsh blow to the leader of the Labour Party, Keir Starmer, who has been on the job for a year.
In a statement, Conservative party spokeswoman Amanda Milling hailed a "historic result."
Speaking to the BBC on Friday morning, Labour's shadow communities secretary Steve Reed said it was "absolutely shattering to see a Conservative MP elected in a place like Hartlepool after nearly 50 years."
"I think 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."
Vote counting was still underway in other parts of the UK after polling stations closed on Thursday evening.
British voters headed to the polls on Thursday to elect new mayors in London and Manchester, new members to the Scottish and Welsh parliaments and hundreds of local councillors.
Due to COVID delaying the process, the results are due to come in on Saturday and Sunday.