When Johnson announced his intent to speak, some commentators wondered if his address might be something akin to the 1990 resignation speech by Geoffrey Howe that preceded the downfall of Margaret Thatcher as prime minister.
In the end, Johnson's speech didn't appear to be the doomsday speech for May that some had feared. But it did underscore how bitterly divided Parliament is over Brexit and the type of relationship Britain wants with the EU after it leaves the bloc in March.
Johnson didn't attack May directly - he noted her "courage and resilience." His remarks are nonetheless a headache for the British Conservative leader who is trying to rally support for her Brexit plan, which would see Britain remain closely aligned with the EU on trade rules for goods (though not services). Johnson said that May's plan would mean that Britain would have "much less scope" to strike free trade deals around the world.
The latest squabbling over Brexit has renewed speculation about a possible leadership challenge for May, who has had a particularly rough few weeks with high-profile resignations, knife-edge votes in the House of Commons, and criticism of her Brexit plans from both those who want to remain in the EU and those who want to leave.
But it's unclear who would replace her. Johnson still has a star quality in some circles, yet he's not as popular as he once was. He didn't signal an imminent leadership challenge or no-confidence vote today.
During his speech, Johnson repeatedly praised May for her remarks at Lancaster House in early 2017, when she said that Britain would seek a clean break from the EU.
Johnson said that since then, Britain had "dithered" and "burned through negotiating capital" and allowed the Northern Irish border issue "to become so politically charged as to dominate the debate."
In the past 18 months, said Johnson, "it is as though a fog of self-doubt has descended."
