British farmers declared a "crisis" on Monday after days of protests over falling prices that saw milk removed from shop shelves and cows walked up supermarkets aisles.

Farmers' unions held an emergency summit on Monday over low milk and meat prices, in a bid to ease the plight of producers as global good prices drop.

"Obviously the industry is in crisis. There's despair within our members," said Meurig Raymond, president of the National Farmers' Union.

"I've been farming for 45 years and this is the worst I've known, particularly the dairy sector and the lamb sector."

The average price paid to farmers for a litre of milk is just under 24 pence ($00.37), a drop of 25% in a year, while farmers' unions estimate it costs between 30 and 32 pence to produce.

In protest, farmers have descended on supermarkets to buy up their entire stocks of milk in outlets of Morrisons, Asda and Lidl, and have blockaded distribution centres.

Over the weekend, about 70 demonstrators herded two cows into a branch of Asda in the central English town of Stafford in act of protest.

The National Farmers' Union warned that the low prices could mean that British milk products could disappear from shelves.

But supermarkets have rejected the criticism, arguing they are the wrong target.

"There is no connection between the price of milk in supermarkets and the price retailers pay farmers for their milk," said a spokesman for the British Retail Consortium.

"We understand the current frustration of farmers but it is wrong to blame retailers."

Germany and France have also seen protests by the agriculture industry in recent weeks, particularly by dairy producers who have demanded higher prices for goods.