The stage is set for a high-stakes battle over the country's future, with public opinion, parliamentary procedure and the law all part of the arsenal. A look at where things stand and what might happen next:
BRITAIN EYES DESTINATION NO-DEAL Johnson became prime minister last month on a promise to take Britain out of the EU on the scheduled date of Oct. 31, "come what may." He says he wants to leave the bloc with a deal, but only if the EU makes big changes to the divorce agreement it struck with his predecessor, Theresa May. The EU says it won't renegotiate, so chances of the U.K. crashing out without a deal are rising.
By law, unless something dramatic happens, Britain will cease to be a member of the EU at 11 p.m. London time on Oct. 31. Johnson's strategy is to stand firm and hope the bloc blinks, while ramping up plans to leave without an agreement by hiring more border officials, stockpiling medicines and preparing for backlogs of trucks around the major Channel port of Dover.
Many economists and businesses say a no-deal Brexit will cause economic turmoil, and Johnson's critics accuse him of steering the country off a cliff. Former Treasury chief Philip Hammond, who quit just before Johnson took office, said the British government appeared to be setting "the bar for negotiations so high that we inevitably leave without a deal."
WHAT POWERS DOES THE BRITISH PARLIAMENT HAVE?
Parliament has already voted several times — although non-bindingly — against a no-deal Brexit. But lawmakers can't agree on what should happen instead. Some want to leave the bloc with a deal, others to remain in the EU.
Parliament is on a summer break until Sept. 3 but opposition legislators and Conservative lawmakers who are against a no-deal Brexit are holding talks in hope of reaching a common strategy.
Lawmakers have two routes to stopping a no-deal Brexit — replacing Johnson's government or passing a law that bans leaving the EU without a deal.
CAN THEY BRING DOWN BORIS JOHNSON?
Lawmakers are expected to try to oust Johnson once Parliament returns. Jeremy Corbyn, leader of the opposition Labour Party, says he plans to call a no-confidence vote in Johnson's government "at the earliest opportunity when we can be confident of success."
Johnson's Conservative government is vulnerable. A recent loss in a special election reduced its working majority to just one vote. Several Conservative lawmakers apprehensive about the damage a disruptive Brexit could do say they would be prepared to bring down a Conservative government to stop a no-deal exit from happening.
But winning a no-confidence vote would not by itself alter Britain's course toward Brexit. There would then be a 14-day period in which either Johnson or another politician could try to secure the confidence of Parliament by winning a new vote.
Whoever succeeded could then form a new government. If no one can win a vote within the 14 days, a national election would be scheduled.
WILL BRITAIN SEE A NATIONAL UNITY GOVERNMENT?
It's unclear who, if anyone, could win the support of a majority of British lawmakers, a notably fractious bunch.
Corbyn, who heads the biggest opposition party, has written to leaders of other groups proposing a Corbyn-led "temporary government" that would seek to delay Brexit day and call a national election.
The idea got a mixed reception.
Liz Saville Roberts, leader of the Welsh party Plaid Cymru, said she welcomed "the fact that at last Jeremy Corbyn is reaching out." But Jo Swinson, leader of the centrist Liberal Democrats, said the idea was "nonsense" because Corbyn was a divisive figure.
The reactions highlight the problem facing the pro-EU forces. The smaller opposition parties agree on the need to avoid a no-deal Brexit, but don't want to put Corbyn — a staunch socialist with little enthusiasm for the EU — in power. Labour, meanwhile, is likely to oppose a politician from any other party heading a national unity government.
If the opposition remains divided, Johnson could wait out the 14 days then call an election as required, but for a date after Oct. 31. That would mean Britain would leave the EU on a no-deal basis during the campaign period.
CAN BRITISH LAWMAKERS OUTLAW NO
Parliament's other route is to pass a law banning Britain from leaving without a deal and requiring the government to ask the EU for a further delay to Brexit.
This could be tricky, because the government controls the parliamentary timetable and there are limited chances for the opposition to introduce legislation. Johnson's allies have also suggested he could suspend Parliament in the fall if it tries to obstruct Brexit.
But lawmakers have an ally in Speaker of the House of Commons John Bercow, who has said he will "fight with every breath in my body" to ensure that Parliament gets its say on such a momentous topic as Brexit.
The EU has said it will only agree to delay Brexit again if there is a strong reason, such as a British election or a new Brexit referendum. There's support in Parliament for either of those options — but, again, it's not clear whether there is majority backing for a single course of action.
WILL THE QUEEN GET DRAGGED INTO BREXIT?
Because Queen Elizabeth II formally appoints British prime ministers, political crises always bring a flurry of speculation about whether the monarch — who is supposed to remain above politics — will get dragged into the Brexit fray.
That chatter increased after Labour economy spokesman John McDonnell said he'd be "sending Jeremy Corbyn in a cab to Buckingham Palace to say we're taking over" if Johnson lost a vote of no confidence.
But it's extremely unlikely the queen would play an active role in deciding who governs. Monarchs once had the power to dismiss British governments but that hasn't happened since 1834.
These days the queen rubber-stamps decisions made by politicians — and those politicians have to sort out the country's political crises.
"The queen acts on the advice of her prime minister and will keep out of things," said constitutional expert Vernon Bogdanor.
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