At last the clean-up begins on the filthy acres that blight the face of Britain

Companies which cause dangerous land contamination will be compelled to detoxify their sites under new laws. In some circumstances they could even be forced to meet the cost of clean-up even though they had long ago sold the land to someone else.

Ministers hope the new legal regime will remove the blight which surrounds tracts of derelict land in industrial areas and inner cities, and thereby reduce the pressure to develop greenfield sites.

The Department of the Environment last week published guidance which is a prerequisite for the new regulations. It covers sites contaminated with toxic chemicals and metals, or where rotting waste produces dangerous levels of the explosive gas methane.

Until now, much of the pollution affecting human health and life from such sites was covered by statutory nuisance laws more than a century old. Uncertainties about who was responsible for cleaning up dangerously contaminated sites, who would foot the bill and what standard of clean- up was required made many developers wary of buying such land and putting it back into use.

The new regulations will compel local councils to survey their areas and identify those sites where there is a significant risk of water pollution or threats to people.

Once they do that - and there is no deadline as yet - the Government will at last have some idea of how much seriously contaminated land there is in Britain. At the moment there are only the vaguest estimates.

But there are believed to be many hundreds of severely contaminated sites covering more than 100 square miles. One of the most detailed large-scale surveys ever done covered Wales and it identified 746 potentially contaminated sites covering 16 square miles - the size of Swansea.

Councils will either reach agreement with whoever caused the contamination on how they should remove the threat or, failing that, order them to do so. If the company or the individual responsible refused, then the council can get the work done itself and claim the money back.

The clean-up will have to be the bare minimum required for whatever use is planned for the site. Thus housing and shopping centre land will have to be detoxified to a higher standard than playing fields or car parks. A company which caused contamination but then sold the land on will not be liable for the clean-up costs if the purchaser knew about the likely problems when the site was bought. Instead, the new owner will take on this responsibility.

Dozens of the contaminated sites which require remediation are expected to be "orphan sites", where whoever caused the contamination cannot be traced, has ceased to trade or cannot afford to pay. Councils will have to rely on central government grants to deal with such sites.

David Cuckson, a solicitor with a City firm who specialises in environmental law, predicted a patchy response from councils under the new regime. "Some are keen to do something about contaminated land, others don't see it as having any priority.

"The problem for councils is that, having made the effort and identified problem sites, they might end up not being able to identify the polluters, or they could plead hardship."