For the past four years, a disaster has been threatening of such calamitous proportions that, should it ever come to term, future generations will rub their eyes in wonder that a Conservative government – a Conservative government – could ever have let it occur.

At stake is much that Tories are supposed to hold dear: the ancient fabric of the country; fiscal responsibility; Britain’s good name. Yet ministers seem content to sit on their hands. As a result, with the clock ticking down towards an October deadline, a grotesque act of vandalism threatens. We are facing a desecration: the desecration that is the Stonehenge road tunnel.

Give it the green light and in a thousand years, when the current political shenanigans are just a footnote in a dusty history book, the most precious prehistoric landscape in Europe will still bear like a scar the mark of our folly and shortsightedness.

As things stand, the plan is to build 4.5km of dual carriageway through a Unesco-designated world heritage site: the Stonehenge landscape.

Some of this dual carriageway, it is true, would be sunk into a tunnel, but much of it would not. The projected tunnel might well enable visitors to admire the stones without the distraction of lorries crawling along the A303 behind them, but the object of their admiration will be the equivalent of an otherwise extinct creature in a zoo. No wonder that Unesco, at the July meeting of its heritage committee, should have registered its alarm, for the scheme ranks as an international scandal.

The landscape that provides Stonehenge with its context, and which is currently enabling archaeologists to trace the ultimate origins of the monument back almost to the original arrival of humans in Britain, will have been devastated. Vast quantities of concrete and asphalt will have desecrated it for all time. The chance to shed a better light on the very beginnings of continuous human settlement in our country will have been lost forever.

Quietly, for the past five months, a planning inspectorate has been sitting in Salisbury, listening to representations on the issue. Midnight last Friday was the deadline for responding to these hearings; 2 October will be the conclusion of the hearings themselves. What has been heard there should be giving ministers very serious pause. It has become clear, from the evidence of government officials themselves, that the £2bn the Stonehenge tunnel will cost – a figure that, as with HS2, seems forever to be rising – is a monstrous waste of money.

Shockingly, based on Highways England’s own figure, it will save just 4.8 seconds per mile on an average 100-mile journey. Adding to the sense of a white elephant on the rampage is evidence heard only last week from a hydrologist, warning that tunnelling through the rock risks seriously affecting the water table. The damage this would inflict on both the archaeology of the Stonehenge landscape and the broader ecology of south Wiltshire’s riverways will be fully known only when it is too late.

Time can be measured in seconds, and it can be measured in millennia. Nowhere else in Britain do the demands of the present and the claims of the past rub up against each other more insistently than on the plain around Stonehenge. The 4.8 seconds per mile saved on an average car journey have to be measured against reaches of time so profound that they shade into the sacred. The sands are running out – but it is still not too late to save the Stonehenge landscape.

Tom Holland’s new book is Dominion: The Making of the Western Mind