IT is a sad day when Zac Goldsmith occupies the moral high ground over the Scottish Government.

The Tory MP’s decision to force a by-election rather than accept the UK Government’s imposition of a third runway at congested Heathrow is being dismissed as a political stunt. But at least he honoured a promise to resign.

That counts for something in an age when hypocrisy has been elevated almost to a principle of government: when the Foreign Secretary writes articles opposing Brexit and then becomes its leading advocate; when the Prime Minister tells bankers privately that leaving the EU will be ruinous and then turns into the apostle of hard Brexit. The attractions of office seem to over-ride not only moral constraints but rational ones, too. Nothing has changed since Mrs May told Goldman Sachs the economy would be seriously damaged by leaving the single market, nor since she pledged to oppose Heathrow’s expansion.

Read more: Nicola Sturgeon's post-Brexit blueprint to be published by end of 2016

She supported David Cameron’s “no ifs, no buts” promise to block runway three. The Scottish Government is also in the dock for hypocrisy. Nicola Sturgeon stands accused of abandoning her environmental principles and succumbing to lobbying from the Chinese-backed consortium behind the Heathrow expansion. The First Minister has apparently placed at Mrs May’s disposal the votes of her 56 SNP MPs at Westminster (technically 54 because two have had to resign the whip) so that she can defeat an expected backbench Tory rebellion.

I don’t know what the rest of the SNP group thinks but those in it should be worried about the reaction in Scotland if they troop into the Tory lobby when Heathrow is put before the Commons, probably next year. After all, the 56 defied Mr Cameron over fox hunting on moral grounds. Environmental issues don’t come much more clear-cut than Heathrow. It’s a funny sense of priorities if foxes are more important than the lungs of west Londoners.

Aviation lobbyists had been spending tens of thousands of pounds installing expensive lounges at the SNP conference but no one seriously believes Ms Sturgeon was easily swayed by lobbyists. So what is going on? Perhaps we will hear in future ministerial memoirs about deals being done with Mrs May behind the scenes. Was Heathrow discussed on the quiet after the Joint Ministerial Committee this week? It’s a coincidence that the announcement was the very next day. The Prime Minister surely couldn’t have made it had she not had the SNP MPs in her pocket, or a reasonable expectation that they would find their way there. With a majority of 12 and around 50 Tory MPs threatening rebellion, the UK Government’s Heathrow arithmetic doesn’t work without SNP support.

Yet SNP MPs could be helping break the law. Heathrow expansion breaches EU regulations on clean air, noise and other forms of pollution. The area around Heathrow is a noxious nitrogen dioxide (NO2) blackspot not just because of the 500,000 planes that fly there ever year, but because of the traffic that piles up around the most congested airport in Europe. Even without Heathrow’s new runway on stilts, the area is on course to breach EU limits on NO2. SNP MPs talk about the malign influence of the metropolis on Scotland but poisoning west Londoners seems to be a rather extreme way of punishing them for it.

Read more: Nicola Sturgeon's post-Brexit blueprint to be published by end of 2016

It also makes a nonsense of the Scottish Government’s commitments on CO2 emissions. We knew that the UK Government had given up on all of this “green crap” as David Cameron put it but we didn’t think the SNP was in the same climate-change camp. Moreover, to reconcile Heathrow with emissions targets, capacity will have to be cut from regional airports. This means even more short-haul flights through London; more business activity focussed on the south east; and more infrastructure billions spent on the city that already receives 24 times more investment per capita than the north of England.

If you were going to build a new airport, Heathrow is the last place you would put it. London is one of the very few major cities in the world where planes routinely fly over the city centre. It’s because of the prevailing winds, which mean they have to approach east to west. I have lived near, but thankfully not under, the Heathrow flightpath and I know what the noise is like. It’s like living under a motorway for planes.

The airport will have to become a no-fly zone for six-and-a-half hours a night so that Londoners can sleep. To make Heathrow’s surroundings tolerable, £700 million will be spent on sound insulation alone. Some 800 homes need to be demolished. The M25 will have to be expanded, according to Heathrow’s own publicity brochures, to a 14 lane mega-highway to cope with the millions of extra passengers trying to arrive at and leave the airport.

So why is the Scottish Government backing Mrs May in this project which many believe is so daft it will never actually be built? Well, they say that 16,000 jobs will be created in Scotland, which is speculative to say the least since it won’t happen for 15 years at the earliest. So, too, is the claim that, in some way, Prestwick airport, which is Scottish Government-owned, will benefit from Heathrow expansion. An industry expert I spoke to who knows a great deal about Prestwick described that as poppycock. This is a dreadfully thin rationale. The Scottish Government has an inconsistent record on the environment, despite its protestations to the contrary. It has sound missions targets but always seems to find money for roads and bridges while GP numbers dwindle and social services are cut.

Read more: Nicola Sturgeon's post-Brexit blueprint to be published by end of 2016

It has opted to cut Air Passenger Duty on the dubious grounds that businessmen will be encouraged to fly to Scotland to save £10 on a business-class ticket. The equivocation on fracking has also caused alarm among loyal SNP supporters. This matters. The Scottish Government can’t afford to be seen as just another tawdry administration that assumes the shape of the last corporate bottom that sat on it.

There seems no obvious reason why the SNP Government had to expend valuable political capital on a Heathrow project that is so obviously damaging. Ms Sturgeon could just have remained aloof, allowed the Gatwick and Heathrow lobbies to fight it out amongst themselves and said, in effect: nothing to do with us. Instead, her Government abandoned its previous agnosticism and its environmental principles and presented SNP MPs with the unedifying prospect of marching into the government lobbies to save an unpopular Prime Minister from defeat.

No one seems to know why she has done it. I don’t believe the First Minister is swayed by the wining and dining of Heathrow lobbyists so the best theory appears to be that she has been offered something in exchange. Well, all I can say is: it better be something pretty damned good.