HS2 was only ever about politics. And the battle will reach the heart of No 10

Will it be yes or no to HS2? Within the next fortnight the answer is due on whether Britain needs a fifth rail pathway to the north from London. At more than £100bn, it would be the country’s biggest peacetime infrastructure project. HS2 no longer has anything to do with trains, let alone economics, politics or the north-south balance. It is about Boris Johnson and what sort of leader he intends to be. That is why HS2 is tearing Downing Street apart.

Related: Grant Shapps delays decision over HS2 rail line

So far, Johnson’s art of government has been to go with the flow, to follow the populist line of least resistance. It has served him well. Out of office two years ago, he opposed HS2. Today the National Audit Office has said, in effect, that the project is beyond anyone’s control. But will the prime minister hold his nerve?

Ahead of the NAO report, there was a flurry of leaks from HS2 acknowledging what was denied last summer, that its costs were like a slowly erupting volcano. Until then, the original £30bn budget had been a fantasy £55.7bn. Then a new HS2 chairman, Allan Cook, decided it was suddenly £88bn. This month that figure mutated to an absurdly precise £106.8bn. Now I gather that figure too is dubious, and a new figure, “with contingency”, is nearer £130bn.

Grant Shapps, the hapless transport secretary, must be close to despair. For 10 years his department has been little more than a public relations agency for a project described to me by one insider as “the most appalling public procurement in modern history”. If this was a local council it would be wound up and the Treasury would take over.

We still await sight of Johnson’s Alice in Wonderland HS2 inquiry, bizarrely chaired by HS2’s outgoing boss, Douglas Oakervee, with his foe Lord Berkeley as deputy. Leaks indicate that even Oakervee is lukewarm. He suggests that 18 trains a day is absurd, and perhaps the links beyond Birmingham to Manchester and Leeds could be postponed. Both would undermine viability.

Problems remain that would have killed a less lucrative rail project. HS2 was never a railway concept. It was a David Cameron whim, sold to him by the former Labour transport secretary Andrew Adonis to show he cared for transport, after he had said he’d cancel a third Heathrow runway. The ill-conceived line to Birmingham would not terminate at any transport interchange at either end. Euston would not connect with HS1 (the Channel tunnel rail link); nor, at the other end, Birmingham’s Curzon Street station, with anywhere but a taxi. At high speed, the trains would guzzle energy and need wide pathways. They would not tilt, so they would actually be slower north to Scotland than ordinary trains. Yet the project would cost the taxpayer as much while building as the entire existing railway. This would be for a tiny elite of premium customers among the 11% of workers who travel by train, most of whom are commuters.

HS2 has been opposed by virtually every parliamentary and official report I have read since 2010. Yet assiduous lobbying by Adonis and the rail consultant Jim Steer has kept it afloat. Aided by the contractors, they have dodged and weaved. First they offered high speed, then they offered capacity. When capacity was clearly more needed elsewhere, and with slackening demand for rail since 2015, they switched again.

Now they are challenging Johnson to show guts, virility and commitment to the north. Civic leaders are summoned to the banner, with Adonis glorying in how many grand projects – from Buckingham Palace to Eurotunnel – have gone way over budget. Today, HS2 is about promoting honest northern enterprise. No matter that studies of high-speed rail elsewhere rarely show benefits flowing to the poorer end of the link. No matter that HS2 would ease the flow of people, talent and money to the London magnet. HS2 is now about image, gesture. Is Johnson a macho northern lion, or another southern mouse?

At this point, argument is for the fairies. Every “northern powerhouse” report pleads for improved local connectivity, both by road and rail. Getting from Leeds to Bradford, Sheffield and Manchester is barely faster than in the 19th century. No commuter crammed on a Leeds platform could honestly demand priority for a premium train from Birmingham to London, built to compete with west coast trains currently running at 60% capacity.

HS2’s most vocal northern supporter, Greater Manchester’s mayor, Andy Burnham, expects an extra £39bn for local and regional connectivity. But he must know that HS2 would suck all oxygen out of railway investment for the next two decades. There is no political vanity for Johnson in a trans-Pennine express, nor in that adrenaline rush of construction lobbyists, to be “shovel ready”. Not a train will run before 2031 at the earliest. The rest of the railway can whistle in the wind. You can’t fool the Treasury all the time.

In Downing Street, battle is joined. Johnson’s top aide, Dominic Cummings, has described HS2 as a “disaster zone”. His transport adviser, Andrew Gilligan, has been an outspoken opponent of HS2 and it is hard to imagine him staying if Johnson reverses his former position. No 10’s mistake so far has been not to fashion a plausible alternative offer to the northern lobby. Burnham and co have been left with HS2 “for free”, or nothing.

Had a similar cash amount been pledged to the northern cities for transport projects of their own choice, not just a fast link to London, this tragic waste might have been different. When I asked the former Manchester city council chief executive Howard Bernstein which priority he would choose, between HS2 and local connectivity, he unequivocally said the latter. But no one offered him that choice. Whitehall does not do local.

Johnson may yet seek a botched halfway house. He could build only the link to Birmingham “for now”, or he could do the opposite and build only the links north from Birmingham. He could halt HS2 in the south at Old Oak Common near London – changing for Heathrow and Crossrail – which would avoid spending billions crashing through Primrose Hill to Euston. It is a measure of the chaos that such wild options are still in play. Either way, the most powerful industry lobby in modern history has already walked away with £8bn of public money, with not a yard of track laid.

What Johnson ultimately decides will speak volumes about his administration. He will need guts to make the right decision; the courage will not be to proceed but to call a halt to this nonsense, to do something better with so much money.

• Simon Jenkins is a Guardian columnist