LONDON — Prime Minister Theresa May stunned Britain on Tuesday by announcing that she would call an early election, placing a bet that voters would give her Conservative Party a strong mandate as her government negotiates the country’s withdrawal from the European Union.
“The country is coming together, but Westminster is not,” Mrs. May said in an unscheduled appearance outside 10 Downing Street, referring to divisions in Parliament. Mrs. May added that she had “only recently and reluctantly come to this conclusion.”
The election would occur on June 8.
Mrs. May had repeatedly ruled out a snap election, so her decision on Tuesday represents an abrupt U-turn.
In calling an early election, she is betting that voters will give the Conservative Party, which holds a slim majority — 330 seats in the 650-member House of Commons — a stronger mandate. The opposition Labour Party is in severe disarray under its hard-left leader, Jeremy Corbyn. Within an hour of Mrs. May’s comments, he said that Labour would welcome an early election — even though many of his critics in the party fear that it will lose seats.
But her announcement is also a huge gamble.
A new election will reopen some of the country’s gravest divisions. It will give Brexit opponents another chance to soften the terms of the withdrawal from the European Union by voting for Liberal Democrat and Labour lawmakers who favor the bloc. It will give the Scottish National Party, which grabbed dozens of seats from Labour in the 2015 national election, a new chance to reissue its call for Scottish independence.
If Western democracies have learned anything over the past year, it is that elections are unpredictable. And if Mrs. May wins anything less than a commanding majority on June 8, she will be weakened.
Many Britons may be exhausted from voting, after a referendum on Scottish independence (September 2014), a general election (May 2015) and the Brexit referendum (June 2016). In addition, local elections are scheduled for May 4. (It was too late to piggyback national elections onto the May 4 vote.)
Mrs. May took power in July; her predecessor, David Cameron, resigned after voters narrowly approved a referendum supporting departure from the European Union, a decision known as Brexit.
Last month, Mrs. May formally initiated the two-year divorce process, one of Britain’s most consequential decisions since World War II.
On Tuesday, she said her decision to call an election was a response to gridlock created by the opposition.
“In recent weeks, Labour have threatened to vote against the final agreement we reach with the European Union,” she said. “The Liberal Democrats said they want to grind the business of government to a standstill. The Scottish National Party say they will vote against the legislation that formally repeals Britain’s membership of the European Union. And unelected members of the House of Lords have vowed to fight us every step of the way.”
She added: “If we do not hold a general election now, their political game playing will continue.”
But her critics were quick to jump on the reversal.
“This announcement is one of the most extraordinary U-turns in recent political history, and it shows that Theresa May is once again putting the interests of her party ahead of those of the country,” Nicola Sturgeon, the leader of the Scottish National Party, said in a statement. “She is clearly betting that the Tories can win a bigger majority in England given the utter disarray in the Labour Party. That makes it all the more important that Scotland is protected from a Tory Party which now sees the chance of grabbing control of government for many years to come and moving the U.K. further to the right — forcing through a hard Brexit and imposing deeper cuts in the process.”
Supporters of an early election had urged Mrs. May to consider the experience of Gordon Brown, a Labour prime minister who took office in 2007 after his predecessor, Tony Blair, stepped aside. Despite polls showing that Labour would win comfortably, Mr. Brown declined to call a general election at that time, and his popularity was badly hurt during the 2008-9 financial crisis.
After 13 years in power, Labour lost the 2010 election, with Mr. Cameron’s Conservatives forming a coalition government with a centrist party, the Liberal Democrats.
That coalition passed a law making it harder to call an early election. Under the law, the Fixed-Term Parliament Act, two-thirds of the House of Commons will have to vote on Wednesday to support an early election. Given Mr. Corbyn’s support for the resolution, that seems likely. (The next regular election would have been due in 2020.)
“Labour will be offering the country an effective alternative to a government that has failed to rebuild the economy, delivered falling living standards and damaging cuts to our schools and N.H.S.,” Mr. Corbyn said in a statement, referring to the National Health Service.
The Liberal Democrats’ leader, Tim Farron, also agreed to support an early election.
“This election is your chance to change the direction of our country,” he said. “If you want to avoid a disastrous hard ‘Brexit,’ if you want to keep Britain in the single market, if you want a Britain that is open, tolerant and united, this is your chance. Only the Liberal Democrats can prevent a Conservative majority.”
Alastair Campbell, who was a senior adviser to Mr. Blair, wrote on Twitter that the state of the post-Blair Labour Party was a factor in Mrs. May’s timing. “With the opposition as it is she thinks she can get away with anything,” he posted.
Mr. Cameron also endorsed the announcement, calling it a “brave — and right — decision.”