LONDON—British Prime Minister Boris Johnson holds a double-digit lead in opinion polls as Britain prepares to vote Thursday in a crucial general election that will determine Brexit’s fate.

If reflected in the election, the lead would be enough to give him a comfortable majority—though a recent narrowing of the lead reflected in some polls is enough to keep the outcome uncertain.

The bitterly fought election campaign bookends more than three years of political stalemate over how—or even if—the U.K. should quit the European Union. If Mr. Johnson’s Conservative Party wins enough votes to form a majority in Parliament it will open the way for the U.K. to leave the EU on Jan. 31.