(Bloomberg) -- Boris Johnson suffered a seismic political upset as his ruling Conservatives lost a key parliamentary election, a result that will heap intense pressure on the U.K. prime minister and may even call his position into question.

The Tories lost Thursday’s vote in North Shropshire in the West Midlands, a rural seat they have held since it was created in 1832. The Liberal Democrats’ Helen Morgan won the by-election with a majority of 5,925 over the conservatives, according to the Press Association, capitalizing on weeks of turmoil and plummeting poll ratings for Johnson and his party.

Pressure from his own colleagues will inevitably now intensify for Johnson to prove he has a grip on government and can still be relied on to guide the party to victory in the next general election due by 2024. It’s an especially sensitive time for the U.K. given the surge in Covid-19 omicron infections that threatens to overwhelm the National Health Service this winter.

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The special election was triggered by the resignation of former minister Owen Paterson after he was found to have breached Parliamentary rules on paid lobbying. Johnson’s attempt to protect him quickly backfired, and led to a damaging row over ethics and intense Tory infighting. 

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The premier has struggled to move past it, and with the Tories falling behind the main opposition Labour Party in national polls he has faced growing questions about his leadership. This week, he suffered his biggest rebellion since becoming prime minister, with 100 Tory MPs opposing parts of his strategy for tackling the surging omicron Covid-19 variant.

The scale of the revolt underlined the depth of anger among Johnson’s MPs, not only at his handling of the pandemic, but over what they see as a series of self-inflicted missteps that began with Paterson.

He represented the largely pro-Brexit seat of North Shropshire for 24 years, and was returned with a thumping majority of close to 23,000 in 2019. On paper, that made it look like an easy hold.

But the Liberal Democrats fought hard to capitalize on the turmoil surrounding Johnson. Last week, they distributed leaflets highlighting reports the prime minister’s staff broke pandemic rules by holding Christmas parties in his Downing Street office, and contrasting the alleged behavior with the reality faced by most people who were forced to stay away from loved ones last year.

Johnson campaigned in North Shropshire himself, along with cabinet ministers Michael Gove and Grant Shapps and dozens of Conservative MPs, as fears grew the Tories could lose the so-called safe seat.

By-elections are known for being used as protest votes, to give the government a kicking between general elections. The concern among Conservative strategists was that many of their former voters would simply choose to stay home, allowing their rivals to win.

Parliament’s Christmas break offers Johnson an opportunity to reset his premiership. Although there is no sign yet of any immediate challenger, history shows that the Conservative Party does not hesitate to move against its leader if they’re no longer seen as a vote winner.

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