LONDON—Boris Johnson, the favorite to become British prime minister, said he was prepared to lead Britain out of the European Union without a deal on Oct. 31 and said any EU attempt to impose trade tariffs would be akin to a Napoleonic-era blockade.
The United Kingdom’s three-year Brexit crisis could be about to deepen as Johnson’s pledge to leave the EU with or without a deal on Halloween could provoke a standoff with parliament, which has indicated its opposition to a no-deal exit.
