Beef and sheep farmers are being forced out of business by low meat prices, the National Farmers' Union has warned.
The union says livestock farmers help maintain fragile landscapes
It says flock and herd numbers have fallen and warns that such a change could threaten the countryside.
Launching its Why Beef and Sheep Farming Matters campaign, the NFU said producers were making a loss on "virtually every animal".
Backing the campaign, celebrity chef Jamie Oliver said it was time for a "call to action" to help UK farmers.
NFU leaders are calling on retailers to work with livestock owners to help them obtain a fair price for their meat.
President Peter Kendall said that low prices had meant Britain's sheep flock had fallen by 24% and its beef herd by 9% during the last 10 years.
"This is mainly due to the unprofitable prices being paid to farmers. This decline must be arrested and reversed if a viable industry is to be maintained."
The union says any further decline in livestock farming will jeopardise some of the UK's most fragile landscapes, because grazing animals help maintain upland areas such as Dartmoor, the Yorkshire Dales and the Lakeland Fells.
Two years ago, the National Trust also warned that hill farming in England was on the brink of collapse.
It said that by 2012 most hill farms would be running at a loss, if they were operating at all.
"Grazing livestock are hugely important to the ecology and fabric of the countryside," Mr Kendall said.
"By not giving our farmers the farmgate price they deserve and need, we are putting at risk the future of British beef and sheep farmers and with that, much of the countryside as we know it today."
British beef 'supported'
The NFU claims retailers can afford to give farmers a higher price for meat.
It says that during the last 27 years the retail price for lamb per kg has risen by £3.82, while farmgate prices have only risen by £1.13.
During the same period, the retail price for beef per kg rose by £2.47, while farmgate prices rose by just 59p, the NFU says.
A spokeswoman for Asda said the supermarket had "a great working relationship" with all of its suppliers.
"Asda is a committed supporter of the British and Scotch lamb sector," she added.
The spokeswoman said Asda had recently been praised by National Sheep Association for its support of British lamb.
A YouGov survey for the union suggested 72% of British adults would prefer to buy British beef or British lamb rather than foreign imports.
The poll of more than 2,000 adults also found that 82% of those asked believed British supermarkets could do more to support beef and sheep farmers.
Jamie Oliver said: "Now is the time for a call to action to help our British farmers. It's been a tough year for them and for many it's just getting worse."