Stars take a shine to Meat Free Mondays
The aim of the campaign is to reduce greenhouse gas emissions from the world's livestock population, thought to be a major cause of global warming.
VIP guests including Kate Bosworth, Kelly Osbourne, Lauren Laverne, Monty Don and Moby walked down a green carpet to the launch event in St James's Park in London.
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Hide AdOno's relationship with John Lennon has in the past been cited in some quarters as a factor in rifts between the Beatles.
But she and Sir Paul were all smiles as they posed together for pictures in the middle of a big group of celebrities.
Addressing his guests, Sir Paul said: "I thought this was a great idea. To just reduce your meat intake maybe by one day a week and this would seriously benefit the planet."
According the United Nations Food & Agriculture Organisation, meat production is responsible for 18 per cent of greenhouse gas emissions compared with an estimated 13 per cent from transport.
Sir Paul's late wife Linda, who died in 1998, was a vegetarian and high-profile campaigner on the issue, and a range of meat-free dishes bearing her name are still being sold.
Yoko Ono said: "Give up one day and then it will be two days maybe. It's a very, very intelligent idea."
And Kelly Osbourne said: "It's just an easy, simple thing to do."
Moby said the idea was about encouraging people rather than pointing the finger.
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Hide AdPressure group Friends of the Earth welcomed the Meat Free Monday idea.
Senior food campaigner, Clare Oxborrow said: "Cutting down on meat delivers a double win for the health of people and the planet."