This was one of the main conclusions from a BBC radio programme broadcast this week.

The programme in the Costing the Earth series looked at the theory often put forward by the vegetarian lobby that if there were no farm animals reared for meat methane emissions would be reduced and it would be better for the climate.

However, the programme showed that growing crops also had an effect on global warming through the loss of carbon by tilling the soil.

It also showed there was a loss of biodiversity without grazing animals.

The programme said that Oxford University's Farm Animal Initiative shows a 50 per cent reduction in birds, bees and butterflies on ungrazed land on its research site.

"So eating meat is perhaps not as bad for the planet as it may at first seem. And surprisingly, some forms of intensive farming score better than extensive," the BBC said.

"Chickens raised indoors are far less environmentally damaging than those allowed to roam free range. That's because they need less feed to keep them warm, and they grow quickly, making them the most efficient converters of food into meat.

"Dairy products from intensively reared cows are also much more climate friendly, as those gaseous emissions from bovine mouths and behinds can be far better controlled by keeping cattle indoors."