LONDON (AP) — Opposition parties are challenging Prime Minister Boris Johnson's insistence that the U.K. will leave the European Union on Oct. 31 even without a deal, setting up a pivotal day in British politics.

Parliament is returning from its summer recess Tuesday with a key piece of legislation on its agenda that would prevent an immediate no-deal exit. If it passes, Johnson's Downing Street office says he'll call an early election.

A no-deal Brexit is considered dangerous because it will sever decades of seamless trade with the single market of 500 million, but Johnson insists the potential for leaving without a deal must remain as a bargaining chip in negotiations with the EU.

The bloc is adamant it will not renegotiate the agreement struck with former Prime Minister Theresa May, which Johnson considers unacceptable.