Taking the piss.

[Citizen's Nephew]

Starmer couldn't ask that question as it's akin to accusing Johnson of lying which is not permissible in the House.

He correctly plays the long game, waiting until today to call on Johnson to resign. If he'd made frequent earlier calls, which would have fallen on deaf ears, the impact would have been negated.

I've just thought about this and I think you're right - it's akin to an accusation and is against the 'rather dated and stupid' Erskine May rules.