"Farmers are going to have to retreat," Simon Harrison of Ireland's University College Cork told reporters at the British Association for the Advancement of Science's annual festival being held at the University of Leicester.

Penny Johnes of the University of Reading said watercourses across Britain were far more heavily polluted than indicated by official government statistics which tended to paint an unrealistically rosy picture of water quality.

"In the last decade of the 20th century, 95 percent of British fresh water was polluted by nutrients - most of which was from diffuse sources - in other words farming," she said.

"British waters are highly degraded in terms of nutrient pollution. We need to change land use," Johnes added.

She said point source pollution from factories or spills was fairly well under control, but that leaching from farmland had barely been touched because it was far more complex and difficult to tackle.

In many instances the problem was due to inappropriate or overly intensive land use that either led to local flooding of towns and villages because flood plains had been drained and rivers diverted or meant heavy run-off of pollutants.

"Current farming methods are not sustainable," Johnes said.

Harrison said the picture was being repeated across Western Europe.

"If you want to have nice rivers there are going to have to be some changes in agricultural practices," he said. "This is a Europe-wide problem."

Johns said farmers were being encouraged by subsidies to over-produce food which was then being dumped on world markets.

What was needed was a new system to encourage them to be custodians of the land rather than plunderers of it.

She said it was not a matter of tearing down industrial agriculture and returning to a pristine past. That was neither practical or desirable.

But setting a water quality benchmark that harked back to the less intensive farming practices before World War Two could be a sensible option.

"We must handle this sensitively. We are only just coming out of mad cow disease and foot-and-mouth, she said.