The letter was sent to 150,000 staff after a tense telephone conversation between Sainsbury's chief executive Justin King and the TV chef earlier this week. Oliver, 32, is paid £1.2m a year to front Sainsbury's advertising campaigns and has been the face of the grocer since 2000.

The chef, who attempted to change the nation's eating habits with a campaign to reform school dinners, has turned his attention to the rearing of intensively farmed chickens.

In a show, Jamie's Fowl Dinners, to be broadcast on Channel 4 on Friday night, he highlights the appalling conditions in which some chickens are kept. In an astonishing attack ahead of the broadcast, he criticised Sainsbury's for refusing to turn up for a public debate on chicken farming.

He told reporters: "It is shocking that the people that I work for did not turn up on the day. I do not know why. The fact that your PR department has not even got the confidence to turn up and talk about what you do ... how dare they not? I was really upset."