The oil painting, called The Byam Family, which dates back to 1764, had been a valued asset of Marlborough College in Wiltshire since 1942. But the public school decided to auction it at Christie's to raise cash for a new performing arts centre and swimming pool.

The masterpiece, due to go for auction on Thursday, sold for an undisclosed sum to an unnamed charitable arts foundation. It has now been donated on long loan to the Holborne Museum at Bath, Avon.

Van Gogh's Sunflowers sold to the Japanese Yasuda Fire Company

The painting depicts a wealthy 18th century family, with George Byam and his wife and daughter surrounded by a stormy landscape. It is considered to be one of Thomas Gainsborough's most important works during the period in which he lived in Bath.

Dr Ann Sumner, the keeper of art at the museum, said: "There was an enormous amount of worry that it could go overseas, but at the last minute it was announced it had been bought by the foundation, which was terribly exciting.

"Now it will be kept at our museum where it can be viewed by members of the public."

The most famous British auction room dismay came with the sale of Van Gogh's Sunflowers, for which the Japanese Yasuda Fire Insurance Company paid �24m in 1987 - the highest price ever paid in London.

Then last year, the painting Van Tromp by Turner, said to be the forerunner of impressionism, was lost to the nation when it was sold to the Getty organisation in the US.