One lesson Boris Johnson must be learning as Prime Minister, if he did not take it well enough as Mayor of London, is that big infrastructure projects have a habit of splitting a politician’s political coalition.

His decision to green-light the first phase of HS2 illustrates this. The move was warmly welcomed by a portion of the Tory tribe, most prominently Andy Street and his campaign to hold on to the West Midlands mayoralty. But it also met serious opposition, including from a section of the Party’s new cohort of northern MPs.

No sooner had he bitten that bullet, however, than he finds another whistling towards him in the form of Heathrow.

The Prime Minister’s record on this largely one of evasion. Whilst Mayor he famously advocated building an entirely new airport in the Thames estuary, which inevitably came to be dubbed ‘Boris Island’. But at least some of his City Hall team take the cynical view that this was much more a device to allow Johnson to simultaneously look pro-expansion whilst opposing every practical means of increasing capacity. Then he rowed back on his pledge to “lie down in front of the bulldozers” when Theresa May graciously gave him the chance to be out of the country during the key Commons vote on Heathrow.

Now he once again finds himself torn between two competing camps. As George Trefgarne noted in a recent piece for this site, many senior London Conservatives are strongly against expansion. Given the upcoming mayoral contest and the Party’s increasingly precarious position in the capital, that is not to be taken lightly.

But this morning’s Daily Telegraph reports that other Conservative MPs are heaping public pressure on the Prime Minister to press ahead with Heathrow expansion, and just as with HS2 there is also a well-established lobby – a ‘Heathrow-industrial complex’, if you like – which has built up around the current proposals.

With HS2, at least, Johnson can shift a lot of the blame to his predecessors. For the minute he could do the same with Heathrow, which was authorised by the previous Parliament. Yet if the Government ends up having to go to the Supreme Court to fight for the proposals, that will remove a lot of that distance and put the Prime Minister in an awkward spot.

Should that happen, it will raise an interesting question: will we hear any more about ‘Boris Island’? After all, the man himself now sits at the apex of British politics. There’s nothing and nobody to stop him at least setting the ball rolling on the idea, and one can see the appeal of building a new, 21st-century air hub which could incorporate a lot of extra capacity, as opposed to alternatives involving relatively small increases.

If we don’t, it probably tells you everything you need to know about whether or not the ‘Boris Bridge’ to Northern Ireland will ever get built.