The first ever census of archaeological sites in England, carried out for English Heritage by Bournemouth University, covered 15,000 out of a total of about 300,000 recorded monuments.

Called Monuments at Risk, the report showed that 16% of all sites have been lost to property development, agricultural activity and natural erosion.

"The report has brought home to me, more than anything else, the way in which our archaeological monuments are being nibbled away," said Professor Timothy Darvill, who headed the survey.

"We need to work really hard, at all levels, to afford our most important monuments the long-term protection they deserve."

This pillbox has now disappeared

For example, one earthwork in Knowlton Dorset which dates back to the early Bronze Age has been partly destroyed by ploughing. Raised earth banks and burial mounds were flattened by farming. Upstanding sections protected as a Scheduled Ancient Monument survive.

A World War II concrete pillbox which once stood at East Mersea near Colchester in Essex has been completely destroyed by coastal erosion.

Only one photograph taken in 1974 remains showing it leaning perilously over a cliff edge.

Rescue plan

Following the survey, English Heritage has published a new strategy called Pathways to Protecting the Past.

It now plans to target monuments in urgent need of protection, commission research into new areas of archaeology on contaminated land and help local authorities develop protect their heritage.

The Monuments at Risk survey used the year 1995 as a benchmark and revealed that:

• there has been a 20% decline in the proportion of archaeological earthworks surviving well - from 95% in 1945 to around 75% in 1995,
• 63% of earthwork monuments are now flat,
• 2% of all mouments are at high risk of serious damage or destruction,
• 28% are at medium risk,
• most of the monuments destroyed were in the south-east of England,
• most of those at risk were in the West Midlands, Yorkshire, Humberside and the North-East.