Our history is crumbling before our averted eyes. That, at least, is the impression given by the Heritage at Risk register launched by English Heritage yesterday. For the past ten years EH has published an annual Buildings at Risk list to spotlight vital historic edifices - everything from castles to collieries - under threat from Man or nature. But the new list is a massive expansion of its early-warning system. The “risk” register has been extended to include monuments (such as burial mounds), historic parks, battlefields and even shipwrecks. More than 5,000 sites are said to be immediately threatened. If ever there were a wake-up call to guard our tangible history for future generations, this is it.

But what threatens heritage? Aren't we now so heritage-conscious that, in some areas, you can't change the knocker on your own front door without planning consent?

Well, there are meddlesome officials in all walks of life, and heritage is no exception. But the dangers facing historic sites are not imagined. Some are in the hands of unscrupulous owners who know they can't demolish Grade I or II* edifices. So they let them disintegrate in the hope that someone else - a trust, or the local authority - will become so concerned that they pick up the restoration bill. And that's not the worst of it. In the past year there has been a big jump in the number of listed buildings mysteriously setting fire to themselves.

Planning pressures put other sites at risk. EH points to the location of the 1643 Battle of Newbury - arguably the turning point of the civil war. If it has new homes built on it (as seems possible, given the housing pressures in Berkshire) we deprive future scholars of any chance of “reading” the battlefield: of interpreting the physical features that determined the battle's outcome and the course of history.

Other sites are threatened by modern agricultural practices. Or, in the case of industrial buildings, by changes in society that render their original functions redundant. And, over the centuries, nature whittles away even the sturdiest of Man's constructions.

So the forces lined up against heritage sites are formidable - and the budget to preserve them tiny. EH's funds can't pay for much more than 1 per cent of the work required. And that's just on the present register. Imagine the scale of the task in 2010, by which time EH intends to expand the risk register to cover all of England's 9,500 conservation areas and its 14,500 listed places of worship (more vulnerable than ever, with the CofE tearing itself apart while pews empty).

One understandable if cold-blooded response to this crisis is: “So what?” Our ancestors, whose handiwork we now bust so many guts to preserve, didn't give a tinker's cuss about their ancestors' handiwork. The Saxons cheerfully pulverised Roman villas, and the Normans did likewise to the Saxons. EH has identified half a million sites - half a million! - that it believes we must fight to preserve. Other lobby groups are fighting to save things like the local pub - 56 closing each month in Britain. Are all these old buildings worth preserving? There must be room for doubt.

Highlighted yesterday, for instance, were two very different sites that have been not just abandoned but desecrated almost to destruction. One is the grandstand of a graffiti-strewn 1930s lido in Uxbridge, West London. The other is a Derbyshire lead mine that's now being used (unofficially) as a tip: someone has even dumped an upturned van in it. Many would argue that if their respective local communities care so little about these places, why should the rest of us be bothered? But that's one reason why EH's register is so vital. Often, local communities and even private owners allow something to rot because they don't realise the historic or aesthetic value of what they have. Pointing this out is sometimes all it takes to launch the conservation crusade.

Sadly there are still too many high-placed but dim people who would rather see old buildings bulldozed than sensitively adapted to new uses, because the latter seems “more trouble than it's worth”. John Prescott's Pathfinder plan to demolish 160,000 perfectly upgradable terrace houses in northern England was the grossest instance of that. To resist such insanity, EH is promoting the notion of “embodied energy” - and it's a good concept. In a world of scarce resources, the argument runs, we should preserve the energy, effort, materials and skills invested in old buildings, even if these resources were deployed centuries ago. Half the waste consigned to UK landfill sites is not plastic bags, dumped PCs or uneaten food - despite the propaganda urging us to recycle household waste. It's the rubble created by the demolition and building trade.

But simply saying “we must conserve rather than demolish” isn't enough. Who will do the conserving? There's a huge shortage of builders, surveyors and engineers with the specialist skills to repair and adapt old buildings. That must be addressed. Why not now? With the housebuilding industry in such doldrums there has surely never been a better time to retrain workers in forgotten crafts and skills.

EH's old Buildings at Risk register had a good record of triggering restoration projects. Back in the mid-1990s, for instance, Birmingham's magnificent 1830s Town Hall looked as dead as a dodo. Despite its fabulous limestone and marble walls, and a façade that looks as if it has been uprooted from Nero's Rome, few Brummies could see any use for it, and it had been closed on safety grounds. But after EH included it on the register the conscience of the city council was pricked, £35 million whistled up from various sources, and the Town Hall gloriously restored. Now it's one of the loveliest (and most used) public arenas in Britain.

That's what can happen when civic pride is reignited. Let's hope that the new Heritage at Risk register does the same for Britain's other shamefully neglected national treasures.

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