People need to give up red meat to prevent catastrophic damage to the planet's climate, a former government chief scientist has told Sky News.
Professor Sir David King, who is setting up a centre for climate repair at the University of Cambridge, said cattle and sheep produce so much greenhouse gas that diets must radically change to stop global warming.
Research shows that beef has a carbon footprint up to nine times higher than the same weight of chicken and around 200 times higher than vegetarian protein such as beans.
Sir David revealed that he has already stopped eating beef and is trying to give up lamb - and said we have a moral imperative to do the same.
"There is no question, we need to do it," he said.
"It is our demand that drives farmers to have those livestock.
"The planet's population is still growing so if you are going to feed 11 billion, perhaps 12 billion people by the end of the century, with a growing middle class, we have got to change behaviour.
"They aspire to live the way we live. If we eat meat it is not setting the right example."
The UK's cattle and sheep produce 21 million tonnes of greenhouse gases every year, largely methane.
Meat consumption has fallen by 3% over the last 20 years, but our appetite for beef has remained level.
Marco Springman, a sustainability researcher from the University of Oxford, said that to keep the planet within "the climate change boundary" people should eat no more than one portion of red meat a week, and one glass of milk, or a slice of cheese, a day.
He said a tax could be a powerful incentive for dietary change.
"We estimate if we were to account for current estimates of climate damage in the prices of food then beef would need to become 20% more expensive and milk 15% more expensive in the UK," he said.
"You can call it putting a fair price on food."
The committee on climate change, the government's official advisers, shy away from recommending big cuts in meat consumption.
It recently produced a report on how the UK could reduce greenhouse gas emissions to "net zero" by 2050.
A 20% reduction in red meat consumption as a result of people adopting healthier diets would reduce greenhouse gases by around six million tonnes a year, according to the report.
But further reductions in consumption would rely on "a larger societal shift with questions over public acceptability".
Instead it argues that a sharp increase in tree planting could offset greenhouse gases from agriculture, as well as aviation.
Both Sir David and Dr Springman told Sky News the offsetting was not enough.
However, innovation could give meat eaters the taste of beef, but with a much reduced carbon footprint.
Sky News was given rare access to a lab in Maastricht in the Netherlands where scientists produced the first burger from cow cells they had grown in a dish. The cells increase in number and coalesce into muscle fibres.
Dr Mark Post, the chief scientific officer of Mosa Meat, showed us a 4g burger containing half a billion cells that had been grown over a period of eight weeks.
But scaled up, with cells grown in enormous tanks, the lab meat could feed millions of people while halving greenhouse gas emissions.
Dr Post said: "This is a way to create exactly the same tissue that we know but without the cow.
"That will reduce the environmental impact.
"We are all decadent people. We want choices in food, but also want to preserve our planet.
"We can satisfy the craving of the hard-core meat eater for meat."