British lawmakers approved the broad outlines of his plan with a larger than expected majority — which was, in itself, a big step forward. But they then voted down Mr. Johnson’s effort to fast-track the legislation, which would force them to scrutinize the 435-page plan in just three days.

In short, the whole Brexit effort has been thrown into a legislative abyss that could mean months of further delays to a process that the nation has long since wearied of.

Quotable: “This is a hell of a big document,” Rory Stewart, a member of the Conservative Party, said while waving a bound copy of the plan.

What’s next: The E.U. has offered a three-month extension, but Mr. Johnson, whose “do or die” Oct. 31 deadline has become ingrained in his public persona, said he would “pause” the legislation and tell E.U. leaders he was not interested in a delay.

He had said he would rather face a general election than ask for a long delay, which could be a negotiating strategy.