On Thursday the high court will rule on a case brought by campaigners against the decision made by Theresa May’s government to allow Heathrow to proceed with its third runway expansion. The decision to expand the airport was both environmentally and economically ill-judged. Obviously, Heathrow Airport Ltd, the company involved, wants the go ahead for its growth strategy – it’s not up to a private company to balance the wider impact of that on the UK economy and people. It’s the government’s job to do that and it has got that balance badly wrong. The Heathrow third runway proposal significantly undermines Boris Johnson’s “levelling up” strategy and it should be stopped.

Of course, it’s a big issue in London – it’s hugely polluting to local communities, causing air pollution in areas that already breach legal limits, extra noise affecting millions of people when they step out of their homes and the obvious public safety risk from extra flights over the most densely populated part of the entire country. Even the Victorians would never have built a national hub airport in such an ill-suited location, surrounded by homes and roads making expansion complex, risky and expensive.

But Heathrow expansion is a national issue too. As the UK’s main hub airport, Heathrow operates like a monopoly. Its expansion actively harms the growth of our regional airports. In 2018 a Department for Transport (DfT) analysis showed that regional airports such as Manchester, Birmingham, Newcastle and Leeds Bradford would have their connections stunted by Heathrow and its third runway. With the Heathrow expansion strategy, extra connections are tilted away from the regional airports and towards London. It means that emerging growth markets for exports end up out of reach and more expensive to access for those British companies outside London. It’s also bad for the regional inward investment needed for the “levelled-up” Britain that the prime minister says he wants. This week’s report by the New Economics Foundation showed that this means a £43bn GDP hit away from other regions and towards the south east and London. Those who say the third runway benefits the whole of the UK are simply ignoring the DfT’s own facts.

And it’s bad for passengers too. Those outside London are consigned to a future of having to trek to the capital’s airport to get flights when they could more easily, speedily and cheaply travel from a local regional airport if the connection were there, worsening the already bad carbon footprint of Heathrow. And all this is out of kilter with an airline industry that is moving point to point and away from inconvenient, expensive hubs which few passengers will want to waste time using when they can connect direct.

Of course the third runway decision was driven by the outdated treasury methodology that skews investment away from the regions and towards the south east – an approach this new No 10 is rightly keen to challenge. But Heathrow expansion was pushed through before Johnson’s tenure so he can and should revisit whether this crucial decision now supports or undermines what he and his government are trying to achieve.

‘Johnson famously said he’d lie down in front of the Heathrow bulldozers.’ Boris Johnson at Heathrow in September 2019. Photograph: Stefan Rousseau/PA

It’s always been a David v Goliath contest for campaigners and local communities against the massively resourced, unaccountable, privately owned and run Heathrow Airport Ltd. Those new “red wall” Tory MPs should not put up with the downgrading of their local airports just so London’s main airport can have its growth plan. Alongside their push for proper railway and roads investment, they should be pressing for a regional aviation strategy too. Some MPs have been promised extra connections from Heathrow, but as the risk to the Newquay-Heathrow route shows, routes that are uneconomic ultimately rely on public subsidy and are unreliable for investors to plan on. Even worse, Heathrow’s costly expansion means this problem will get worse and there will be more unprofitable domestic routes to Heathrow under pressure of being cut over the coming years in favour of more profitable international routes with more passengers. The whole approach is wrong. Our hub airport is expensive and outdated.

I campaigned with both Johnson and Zac Goldsmith, then the local Richmond Park MP, against Heathrow expansion. Johnson famously said he’d lie down in front of the Heathrow bulldozers. He is now prime minister and Lord Goldsmith is an environment minister. They are in the very decision-making roles we were lobbying as campaigners.

As I moved out of my MP office at the end of last year, I came across a note that Johnson, as London mayor, had written to me several years ago. At the bottom was scrawled “Down with 3rd runway!” Now he gets to decide. Instead of leaving it to a court, this government should take a decision. It’s time to ditch the bulldozers, ditch the third runway and have the regional airports strategy that a levelled-up Britain needs instead.

• Justine Greening was Conservative MP for Putney from 2005-2019 and is a former cabinet minster