LONDON—Fresh from a decisive electoral victory, U.K. Prime Minister Boris Johnson pledged to move quickly to take Britain out of the European Union, while promising to deliver billions of pounds in public spending to consolidate the Conservative Party’s once-in-a-generation gains among working-class voters still hurting from the financial crisis.

The Conservatives won 364 seats in the U.K. Parliament with just one seat yet to be declared, the party’s biggest win since former Prime Minister Margaret Thatcher’s 1987 re-election.

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