He shunned honours and refused to have an entry in Who’s Who but became one of Britain’s most generous philanthropists. Now, a year after his death, it has been revealed that Simon Sainsbury has made one of the most significant art bequests to the nation, a collection worth about £100 million.

The Tate and National galleries have received 18 paintings, including works by Monet, Degas, Gauguin and Bacon, each one a masterpiece, from a benefactor who preferred anonymity during his lifetime.

The Tate gets 13 works by Bacon, Balthus, Bonnard, Freud, Gainsborough, Pasmore, Wootton and Zoffany, while the National has five by Degas, Gauguin, Monet and Rousseau.

Sainsbury, a great-grandson of John James Sainsbury, who opened his first shop selling butter, eggs and milk at 173 Drury Lane in 1869, lavished millions of pounds on the two institutions during his lifetime.

During the last ten years of his life he had told them that he wanted the pick of his international and British art collection to be enjoyed by the public after his death. Some of the works have rarely, if ever, been exhibited.

The rest of his collection has been left to his partner, Stewart Grimshaw, with whom Sainsbury shared his life and homes in London and West Sussex for 40 years, celebrating a civil partnership last year. Sir Nicholas Serota, director of the Tate, said: “Simon was one of the UK’s most private but generous philanthropists. I am extremely grateful that he chose to bequeath so many remarkable works to the nation.

“This is one of the most important gifts in the history of Tate . . . what a difference they will make to our collections.”

Sainsbury had supported institutions from the National Theatre to the Royal Opera House and also a great many smaller, less glamorous appeals. His insistence on anonymity became impossible when the three Sainsbury brothers: Simon, Lord John and Sir Timothy, came together to fund the National’s Sainsbury Wing, which opened in 1991 at a reported cost of £50 million.

Works donated to the National include Monet’s Snow Scene at Argenteuil, 1875. This canvas is the largest and most atmospheric of about 18 scenes that he created during the winters of 1874-75. Donations to the Tate include Bacon’s powerful and quintessential Study for a Portrait, 1952, created when the artist was developing his iconic Screaming Pope series.

Sir Nicholas said that the Tate now had one of the premier collections of Bonnards outside Paris with the addition of the luminous Nude in the Bath, 1925, and The Yellow Boat, c 1936-8.

Three paintings by Freud, arguably Britain’s foremost contemporary artist, include Girl with a Kitten, 1947, a striking image of the artist’s first wife, and Boy Smoking, 1950-51, a haunting image, despite its small size.

Asked what the collection said about the collector, Sir Nicholas said: “He had a very good eye, with a particular love of paintings that showed domestic interiors and individuals within those interiors. He was an incredibly sensitive person . . . his early British pictures reflect an interest in the English landscape and sporting pastimes.”

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