Voting was closing Monday in the race to become Britain's next prime minister, as critics of likely winner Boris Johnson condemned his vow to take Britain out of the European Union with or without a divorce deal.

Boris Johnson, a leadership candidate for Britain's Conservative Party, reacts during a hustings event in Nottingham, Britain, July 6, 2019. (Photo: VCG)

Members of the governing Conservative Party had until 5 p.m. to return postal ballots in the contest between Johnson and Jeremy Hunt to lead the party.

The winner will be announced Tuesday, and will take over as the nation's leader from Prime Minister Theresa May the following day.

Johnson, a populist former mayor of London, is the strong favorite.

Several members of May's government have said they will resign before they can be fired by Johnson over their opposition to his threat to go through with a no-deal Brexit if he can't secure a renegotiated settlement with the EU.

Most economists say quitting the 28-nation bloc without a deal would cause Britain economic turmoil. The U.K.'s official economic watchdog has forecast that a no-deal Brexit would trigger a recession, with the pound plummeting in value, borrowing soaring by 30 billion pounds ($37 billion) and the economy shrinking 2% in a year.

Former Labour Prime Minister Gordon Brown said Monday that a no-deal Brexit would be "an act of economic self-harm that runs wholly counter to the national interest."

EU leaders insist they won't reopen the 585-page withdrawal agreement they made with May's government, which has been repeatedly rejected by Britain's Parliament.

Foreign Office Minister Alan Duncan quit Monday, lamenting in his resignation letter that "we have had to spend every day working beneath the dark cloud of Brexit."

Other government ministers, including Treasury chief Philip Hammond, are set to resign on Wednesday.

The new prime minister will preside over a House of Commons in which most members oppose leaving the EU without a deal, and where the Conservative Party lacks an overall majority.

Opposition parties are preparing for an early election which could be triggered if the government loses a no-confidence vote in the coming months.

The centrist Liberal Democrats, who have seen a surge in support thanks to their strongly anti-Brexit stance, were set to declare the winner of their own leadership contest on Monday.