Serious water pollution incidents increased by 50% in England and Wales last year with farmers responsible for more than a quarter of them, the Environment Agency says.

The organisation's annual pollution report is expected to show that industry as a whole is continuing to get cleaner but that there are worrying exceptions.

The most serious incidents of water pollution increased in number from 77 in 2000 to 118 last year, bucking the recent trend.

There is particular concern about pollution from farms, which accounted for 27% of the most serious offences.

In one incident, silage waste leaked into a river in Leicestershire.

This wiped out the river's population of native brown trout, which cannot be replaced because of their unique genetic make-up.

The farmer involved was fined �1,600 plus �2,000 in costs.

The Environment Agency is expected to repeat its claim that fines imposed by the courts often do not match the environmental damage done.