Theresa May reprimands Johnson

Theresa May arrives in Canada Image copyright PA

"This government is driven from the front."

It wasn't the most vicious reprimand. Boris Johnson and many of his contemporaries will have heard and perhaps felt far worse at Eton College.

But the meaning was clear enough. "I'm in charge," Theresa May was saying, "and not Boris".

Bout of speculation

The prime minister's assertion in plain language that ways to spend money saved by the Treasury from Brexit would be taken by the government, by ministers collectively (and not simply by Boris writing a 4,000 word essay for a newspaper with little or no prior consultation with Number 10) sounded like a second sharp little slap.

The foreign secretary's intervention has set off a new bout of speculation about power play and tension in government, even paper talk that Mr Johnson might contemplate a sudden and hugely disruptive resignation.

On the flight to Ottawa, senior Downing Street staffers seemed to feel such talk was overblown.

The version preferred by Team May is that Mr Johnson simply felt the need to make his presence felt in his recently-adopted Brexit cause. They deny that he'd been marginalised in discussions about the major speech planned by Theresa May and scheduled for Friday in Florence.

Damian Green told me, on BBC Radio 5 live's Pienaar's Politics programme, that speech would rank among the most significant Mrs May has delivered so far.

Valuable time

So the timing was dreadful, and the foreign secretary's intervention looked to many like a pre-emptive strike, delivered by a man keen to revive his dreams of leadership.

Just three weeks ago, the prime minister's trade trip to Japan was overwhelmed by speculation about her chances of surviving in Downing Street much beyond Brexit. She tried to end the chatter, and achieved the opposite.

Mrs May has been burning up a great deal of valuable time - and a fair amount of RAF fuel - trying to settle nagging questions about how securely she is sitting in her own study in Downing Street.

Maybe she can use her talks in Canada, her presence at the UN in New York and more importantly her speech in Italy to restore order, and convince the many leaders she'll meet this week that she is truly in the driving seat.

Maybe. But for any prime minister at any time, having to assert that he or she is genuinely the one working the pedals and gripping the wheel is scarcely a sign of strength.