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.