LONDON—An early exit polll suggests British Prime Minister Boris Johnson is set to win a large majority in Thursday’s general election, an outcome that would mark a stunning victory for the Brexit cheerleader and pave the way for the U.K. Parliament to trigger a long-awaited split with the European Union.

The exit poll commissioned by three national broadcasters, released just after polling stations closed Thursday, said Mr. Johnson’s Conservative Party was on track to win 368 seats in Britain’s 650-seat House of Commons. The main opposition Labour Party was forecast to win 191, with smaller parties taking the rest.

The poll asks roughly 30,000 people how they voted and has correctly forecast the outcome of two out of the last three elections and the 2016 Brexit referendum. Its organizers say they should be able to predict the seats won by each of the main parties to within 10 or 15 seats.

Mr. Johnson’s expected triumph came on the back of a simple message that a vote for the Conservatives would “Get Brexit Done.” It was supplemented by promises of extra government spending to reverse some of the effects of a decade of public belt-tightening that followed the financial crash.

The scale of the projected victory all but ensures that Parliament will vote to leave the EU at the end of January, ending a three-year political standoff over how and whether the U.K. should break with its biggest trading partner.

The larger the majority Mr. Johnson has, the greater leeway he will have in Parliament to steer future trade talks with the EU in any direction he chooses. Read more at WSJ