THERESA May has finally put her foot down on Brexit.

After Boris Johnson’s “back-seat driving” antics, the Prime Minister declared that “this Government is driven from the front”.

So what is the Foreign Secretary up to? His unauthorised 4,200-word article appeared just days before Mrs May is due to travel to Florence to make a keynote intervention with a speech on the post-Brexit transition period.

Money, it seems, is at the root of Mr Johnson’s concerns.

Word emanating from Whitehall is that Chancellor Philip Hammond and Brexit Secretary David Davis are pondering the idea of pouring £10bn a year into EU coffers to ensure Britain maintains easy access to the European single market. Over three years, this would be £30bn. This, the hope is, would avoid the Brexit cliff-edge and help smooth the way to a future trade deal with our continental neighbours.

But Boris and the hardline Brexiteers believe Britain need pay not a penny for access to the European market; so his article is clearly perceived as an overt manoeuvre to put pressure on the PM.

Knowing that Mrs May is now in a delicate and weak position, Mr Johnson feels confident he can shout his instructions to make sure the PM keeps on the hard Brexit highway, safe in the knowledge that the Tory leader could not sack the Government’s No 1 Brexiteer; better to have him in the car, asking “are we there yet” than having him throw stones from the pavement and become a magnet for the disaffected.

Yet, the word at Westminster is that the Secretary of State is intent at some appropriate point to jump the car and cry “betrayal”.

The thinking is that Mr Johnson will, at the right moment, position himself on the outside, so that when the Maymobile careers off the road, it will be the London MP, untainted by all the zigging and zagging of the Brussels negotiations, who jumps back into the driving seat to steer the vehicle towards his “glorious Brexit”; possibly with no deal at all come 2019.

Of course, Mrs May can only hope that by putting her foot down now, the back-seat driver shuts up and endures the ride towards Brexit and the post-2019 transition.

On the other hand, given all the uncertainty, putting her foot down might just mean Mrs May hits the accelerator towards a Brexit car crash.