The BBC is facing a barrage of criticism after forcing one of its most senior newsreaders to step down as patron of a charity that supports farmers in the developing world.

The corporation has told George Alagiah, presenter of the Six o’Clock News, that he must leave his unpaid position at the Fairtrade Foundation because of impartiality rules, although he has held the post for seven years.

Alagiah has written to supporters of the charity, which promotes fair dealings with farmers in the developing world, to express his frustration and yesterday the chief executive of Sainsbury’s, the first major supermarket to stock Fairtrade produce, said he was perplexed by the BBC’s decision.

The corporation, which has refused to provide the Fairtrade Foundation with a formal reason for the move, even trumpets Alagiah’s position in his biography on the BBC website.

In a letter published in the new summer edition of the Fairtrade Foundation’s magazine, Alagiah wrote: “It is with great regret that I have to tell you that this will be the last time I write as your Patron, and I want to explain why I am no longer able to continue in this role. The decision is not mine.

“My senior colleagues at the BBC have come to the conclusion that being the official patron of the Fairtrade Foundation is no longer compatible with being a high-profile journalist in BBC News. They believe that Fairtrade represents a potential conflict of interest which could undermine my impartiality.

“In the many years that I have been your patron there has not been a single complaint (that I am aware of) to the BBC, so you can imagine how taken aback I was by the decision.”

The BBC will not disclose why, after seven years, it is now inappropriate for Alagiah to continue, but sources at the corporation said the decision was linked to a forthcoming BBC2 series about food that he is to present.

Justin King, chief executive of Sainsbury’s, called on the BBC to reconsider, adding: “I was perplexed to hear that the BBC has required George Alagiah to step down from his role as patron of the Fairtrade Foundation.”

A spokeswoman for the Fairtrade Foundation said: “We are extremely disappointed by the BBC’s decision and have had no formal explanation.”

A BBC spokeswoman said: “The BBC has guidelines in place to ensure our reputation for impartiality is protected. That is paramount.”

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