Michael Shanks's win in Rutherglen and Hamilton West returned the seat to Labour after it was won by the SNP in 2019.

Speaking after the win this morning, Sir Keir said: "We've changed. And because we've changed we are now the party of change here in Scotland. We're the party of change in Britain, we're the party of change right across the whole country."

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Speaking to the press afterwards, the Labour leader said: "I don't think anybody will look back at this and think it's anything other than a historic day for Labour in Scotland."

He added to a rally: "They said that we couldn't change the Labour Party and we did it.

"They said that we couldn't win in the south of England and the north of England, and we did it. They said 'you'll never beat the SNP in Scotland' and Rutherglen, you did it.

"When I left here a week ago with the team, I said 'you've got to win it'. You blew the doors off!"

Mr Shanks replaces the former SNP MP Margaret Ferrier, who was ousted from her seat in August following a successful recall petition for breaching COVID restrictions during lockdown in 2020.

Created in 2005, the seat was held from then until 2015 by Labour, before going to the SNP. It was won back by Labour in 2017 before flipping back to the SNP in 2019.

It is the first Westminster by-election loss for the SNP, with a 20.4-point swing.

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As Sky election analyst professor Michael Thrasher points out, it was "much worse than had been expected from recent Scottish-wide polling, which suggested a swing of half this size".

He also notes that "history shows that Labour needs a strong presence in Scotland to help drive electoral success".

But turnout was low, with just 30,531 votes cast from an electorate of 82,104 - meaning just 37.19% of those eligible took part in the election.

This compares to the 53,794 - 66.48% - who cast ballots in the 2019 general election.

Labour secured 17,845 votes, compared to the 8,399 won by the SNP.

Humza Yousaf, the leader of the SNP, said that while the "buck stopped" with him, he sought to highlight Ms Ferrier's "reckless actions" and the "very difficult contextual factors" around the vote.

Asked if he was facing a leadership crisis, Mr Yousaf said: "No, I don't think so."

Stephen Flynn, the SNP's leader in Westminster, said his party cannot carry on with "business as usual" following the election loss.

"It has obviously been a very tough night for the Scottish National Party," he told the BBC.

"We've had long-standing challenges in relation to Margaret Ferrier and her conduct in terms of breaking coronavirus rules, we've obviously had a lot of internal differences over recent months.

"It is important for us now to reflect on the scale of the defeat that we have had, to look internally at what's gone wrong and to see what we can do better in future to ensure that we win this seat back in the general election next year.

"We have lost by a significant margin, we know that our vote has not turned out, and we know that we need to remotivate people to vote for the Scottish National Party.

"Ultimately, the Scottish National Party has unfinished business … and I want to get on and ensure that we are an independent nation before too long.

"The party needs to recognise it can't just be business as usual. I don't think anybody is going to say it should just be business as usual."