ROAST beef and diary products may have to disappear from the British diet if the country is to meet its pledge to cut carbon emissions by 80%, a government report has warned.

It found that the greenhouse gases generated by agriculture, and especially by Britain's 10.5m cows, will seriously undermine any attempt to meet the targets.

The report, from the Parliamentary Office of Science and Technology, warns that farming generates greenhouse gases equivalent to 65m tonnes of CO2 a year.

At the moment this is quite small compared with the 654m tonnes of CO2 Britain emits in total.

However, the new Climate Change Act means the government is legally obliged to cut emissions to 156m tonnes by 2050.

This, the report warns, can only be achieved if there are massive changes in British farming.

It said: "There is potential to reduce emissions but it is difficult to realise without reducing livestock numbers."

The report coincides with the latest global climate negotiations in Poznan, Poland, this week, where 180 countries are trying to agree a replacement for the Kyoto treaty on limiting greenhouse gases which expires in 2012.

Paul Dodds, author of the report, said: "The problem with cows is that they generate exceptionally large amounts of methane, a gas which has more than 20 times the global warming impact of CO2. Britain's 8.4m beef cattle and 2.1m dairy cows generate the equivalent of 18m tonnes of CO2 each year."

It means that, by 2050, Britain's beef and dairy industry would be consuming more than 11% of the nation's CO2 allowance - a level Dodds regards as unsustainable.

He suggests consumers will have to switch to more vegetable-based diets with most meat coming from pigs, chicken and sheep, which generate lower levels of greenhouse gases. Exactly how the government might stop people eating roast beef is uncertain but the most likely option is putting a carbon tax on cows.

Among politicians there is growing concern over whether Britons will accept the huge lifestyle changes needed if the country is to cut emissions on the scale envisaged. The government faces growing controversy over its plans to build a third runway at Heathrow and expand most regional airports.

The runway - which would be ready by 2020 - would take Heathrow's capacity from 480,000 flights a year to more than 700,000.

Last week, Geoff Hoon, the transport secretary delayed the decision on the runway for the third time this year. A report commissioned from Arup by the Department for Communities and Local Government to advise on what policies will be needed to achieve the 80 percent cut in emissions recommends a dramatic rise in London's congestion charge and an extension of the zone to cover the whole of Greater London, as well as parts of Kent and Essex. In return travellers would be provided with a free, extensive bus network.

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