For the second time in three years, Boris Johnson, a politician whose ambition and superficial charm far outstrip his ability, judgment or principles, is destabilizing the British government and threatening the country’s future.

On Monday, Mr. Johnson, in protest against Prime Minister Theresa May’s plans for Brexit, resigned from his post as foreign secretary. Now Mrs. May’s authority, longevity and ability to deliver a Brexit without causing an economic crisis are in question. But further political paralysis seems certain.

Britain is in this mess principally because the Brexiteers — led largely by Mr. Johnson — sold the country a series of lies in the lead up to the June 2016 referendum on leaving the European Union. They did so because neither Mr. Johnson nor his fellow leader of the Leave campaign, Michael Gove, intended, wanted or expected to win.

At the start of 2016, Mr. Johnson was perhaps the most popular politician in Britain. Supporters and fans mobbed him at train stations and traffic lights; pollsters and pundits thought he could reach the parts of the country that other Conservatives could never touch. But he was also driven and insecure, so desperate to guarantee he would be the next prime minister that he cynically abandoned his own previous positions on the European Union in order to try to secure support from his party’s Euroskeptic right wing.

Because Mr. Johnson and Mr. Gove were confident that the Leave campaign was a hopeless cause, they were free to make ridiculous claims that they had no expectation of ever having to fulfill. They said that Brexit would make Britain both richer and more independent, with more money for the National Health Service, much greater control of immigration and continued friction-free trade with Europe.

Every earnest warning from the other side — about how any Brexit would damage trade, business and jobs — was dismissed airily by the Brexiteers. There were no costs or downsides in this vision of the future.

This casual dishonesty has had devastating consequences.

In the two years since the Leave campaign unexpectedly won, nobody, from the prime minister to Mr. Johnson to the Labour Party, has been able to come up with a plan for exiting the European Union that can satisfy both a majority in Parliament and the expectant public. Why? Because fulfilling the false promises peddled by Mr. Johnson during the campaign is impossible.

The gulf between the easy, prosperous, productive Brexit that its voters are impatiently expecting, and the grim, complicated cost of disentangling economies that have been intertwined for decades has poisoned and paralyzed British politics.

The Conservatives’ leaders cannot admit to the electorate that they were deceived without splitting the party. And instead of apologizing for misleading voters, Mr. Johnson and the other Brexiteers have doubled down, taking refuge in optimistic slogans and vapid promises, refusing to believe the increasingly agitated evidence from hospitals, airlines, farmers, supermarkets and factories that a hard Brexit will damage them all.