LONDON — No date has yet been set, and much of the country is seemingly on vacation, but on Monday unofficial campaigning for Britain’s next general election was well underway.
In a speech, the opposition leader, Jeremy Corbyn, promised to do “everything necessary” to stop Britain’s exit from the European Union without an agreement. Mr. Corbyn also derided what he called Prime Minister Boris Johnson’s “fake populism,” and described him as Britain’s version of President Trump.
That comparison has been made by the president himself, but it is a mixed blessing in a country where Mr. Trump is deeply disliked by many voters. And it suggested that Mr. Corbyn is, perhaps a little belatedly, preparing for an election many analysts expect in the fall.
“Johnson is Britain’s Trump, as the U.S. president himself declared — so it must be true, it cannot be said to be fake news,” Mr. Corbyn said in a speech in Corby, Northamptonshire, that rehearsed a number of lines of attack against Mr. Johnson, accusing him of ties to hedge funds, bankers and a wealthy elite.
That criticism from Mr. Corbyn followed the leak on Sunday of a government document that warned a no-deal Brexit could mean shortages of fuel, food and medicine, jammed ports and a hard border in Ireland.
Mr. Johnson’s supporters have reportedly blamed the leak on ministers who wish to remain in the European Union and wished to sabotage his efforts to negotiate a new and improved withdrawal agreement. Mr. Johnson’s office dismissed the leaked document as outdated.
But the fury of the response to the leak underscored the sensitivity of its contents.
Since becoming prime minister Mr. Johnson has given every indication he is preparing for a general election, because his working majority in Parliament is down to just one lawmaker.
The big question is whether Britons will vote before or after their country has left the European Union, a step set for Oct. 31, and therefore whether they will shape one of the most important decisions in recent British history.
Mr. Johnson has consistently held that Britain would leave the bloc on schedule, without any agreement if necessary. He insists that lawmakers — a majority of whom oppose such a step — cannot stop the departure even if they pass a vote of no confidence in the government next month.
Under those circumstances an election could be delayed until November — after Britain had left without any agreement — effectively bypassing Parliament, according to Dominic Cummings, an influential aide to Mr. Johnson.
Whether this is a bluff is perhaps the central issue of British politics.
In a letter to Donald Tusk, the European Council president, Mr. Johnson on Monday hinted at some new flexibility if the bloc were to grant his central demand by dropping the so-called Irish backstop, a plan to avoid the need for checks on goods at the Irish border after Brexit.
If it did so, Britain would look “constructively and flexibly at what commitments might help” to keep the border free of checkpoints if a technological solution had not been found by the end of a transitional period, Mr. Johnson wrote.
But so far the European Union has refused to reopen the issue of the backstop, which is contained in a withdrawal agreement negotiated with Theresa May, the former prime minister.
Mr. Johnson says he can secure changes to the withdrawal agreement — rejected three times by Parliament already — only if other European leaders believe Britain is prepared to risk a no-deal Brexit. So ramping up preparations for no deal, and making it sound likely, would be logical even if he did not want that outcome.
Yet faced with competition from the Brexit Party led by Nigel Farage, who would be content with a no-deal Brexit, Mr. Johnson also has said it would be disastrous for his Conservative Party to risk an election before Britain has quit the bloc.
That underscores the difficult options facing Mr. Johnson.
Electoral support for governments generally drops sharply after disruptive events, said Anand Menon, professor of European politics and foreign affairs at King’s College London, making a post-Brexit election in November highly risky for Mr. Johnson. “It would be very brave to take us out with no deal and then have a general election,” he said.
“He needs to have a general election before if he wants no deal,” Mr. Menon said.
Mr. Corbyn may have reached that conclusion after having ceded much of the political stage to Mr. Johnson in recent weeks, in which the prime minister has improved Conservative opinion poll ratings.
Though Mr. Corbyn surprised many in 2017 by leading an effective general election campaign concentrating on austerity, his task next time will be harder.
Mr. Corbyn’s leadership has been tarnished by persistent claims that he tolerated anti-Semitism within the Labour Party. And Labour is divided over Brexit, the question likely to dominate the election if it happens before the departure date, and in particular on whether to hold a second referendum about leaving.
On Monday, Mr. Corbyn said Labour would commit to holding a public vote to give the electorate the final say, with credible options for both sides, including the possibility to remain. But he refused to commit himself to campaigning to stay in the bloc.
Mr. Corbyn outlined his opposition to leaving without any deal, and appealed to lawmakers from other parties to support a looming motion of no confidence in Mr. Johnson’s government.
Mr. Johnson will make his first international outing as prime minister this week, visiting Berlin to meet Germany’s chancellor, Angela Merkel, and then Paris to meet France’s president, Emmanuel Macron.
Neither side is predicting a big breakthrough.
Mr. Johnson is expected to repeat demands that the European Union scrap the so-called Irish backstop, a plan to avoid the need for checks on goods at the Irish border after Brexit.
In turn, the German and French leaders are likely to restate their insistence that the withdrawal agreement negotiated by Mr. Johnson’s predecessor, Theresa May, including the backstop, cannot be reopened.