Environmental health chief Peter Ramsdale says further tests will be needed to find out whether the gas and liquid escapes are significant.

If they are, it could cost millions of pounds to put right – money which the council says it does not have.

Tackling contamination, pollution and other health issues relating mainly to former refuse tips has been made high priority by the council.

Five former tips are regularly monitored for possible gas emissions and particular attention is being paid to the former Scout Road tip, at Mytholmroyd, which was also used as an asbestos waste dump.

Making safe a similar tip at Pecket Well, Hebden Bridge, where waste from the notorious Acre Mill asbestos factory was dumped cost more than £2.5 million.

"A risk assessment initially showed a total of 6,457 sites of concern belonging to the council and the private sector," said Mr Ramsdale, in a report to the council's health and social care panel.

"This resulted in 1,257 sites with pollutant leakage but to identify whether this is significant, it will be necessary to undertake detailed site examinations and risk assessments.

"The remaining 5,200 sites might also be contaminated but because there is no pollutant leakage, they do not pose a risk."

He said getting to grips with the problem has been slow because contaminated sites often involved large areas of land and some contained deadly asbestos waste.

"The engineering solutions are complex, as are the health and safety issues."

Most of the work done to cure contamination problems is done when developers submit plans to find new uses for the land.

During 2006/7, environmental health officers were consulted on 737 schemes involving contaminated land and 63 of those schemes were considered inadequate.

The scrutiny panel will be told on Wednesday the council does not have a budget for sorting out contaminated land and relies totally on Government grants.