LONDON — British lawmakers, capping what may be one of the most abysmal starts any British leader has ever endured, on Monday rejected Prime Minister Boris Johnson’s bid to hold a new national election.

For Britain’s bare-knuckled new prime minister, it was a day of defeat. Parliament’s rejection of a snap election came as a new law went into effect on Monday blocking Mr. Johnson from pursuing a “no deal” withdrawal from the European Union.

Parliament is now suspended until mid-October, the result of earlier political maneuvering by the prime minister. But by Monday’s end, it seemed clear that if Mr. Johnson had thought he could outfox Parliament by suspending it, sidelining lawmakers at a critical moment in the Brexit debate, he was the one who had been outmaneuvered.

Now, the man who promised to deliver Britain’s withdrawal from the European Union “do or die,” formal withdrawal agreement or not, is suddenly flailing for a new strategy.