A shy spinster who spent her life amassing an extraordinary collection of fine watercolours worth millions of pounds has bequeathed them to the nation.

Dorothy Scharf was so reticent that she relied on her mother, who lived next door, to communicate on her behalf.

The collection of paintings that she built up until her death two years ago has astonished the art world. Eight masterpieces by Turner, along with paintings by Gainsborough and Constable, are among 51 important watercolours that she has left to the world-renowned Courtauld Institute of Art Gallery in London. The donation is the most significant single addition to the Courtauld in more than a quarter of a century.

The works were created between 1750 and 1850, the golden age of British watercolour painting.

The Courtauld had been completely unaware of Scharf’s final wishes. Dr Ernst Vegelin, its head, was taken aback when he heard the news and overwhelmed when boxes of watercolours started arriving: “There was a sense of revelation as the boxes opened. It was wonderful.”

Little is known about Scharf beyond that she spent some years in Australia and that her grandfather was an art dealer, but not a well-known one. Although comfortable financially, she was by no means wealthy. She and her mother lived in two neighbouring flats in London, which they filled with her watercolours.

Scharf had a natural eye for art, researching anything that caught her attention. Many of her works were bought from, or with the advice of, a small number of London dealers.

Andrew Wyld, former director of the watercolour department at Agnew’s, a leading Bond Street gallery, advised her during the 1990s and remembers her as an extremely shy and lonely woman. “I think of her collection as being her voice,” he said. “She found it difficult to communicate with most people verbally.” But she was also one of the most astute and demanding collectors he has encountered in 35 years of art dealing.

He said: “Communication, including fierce negotiation, was often carried on between us via her mother. Once the price was right, and details of attribution and provenance agreed, a sense of calm would descend.” That way, Scharf built up an astonishing collection. The eight Turners include the atmospheric view Seelisburg by Moonlightand the vivid Margate Pier. Other jewels include Constable’s Stanway Mill, near Colchester, an intensely coloured image by a master of British landscape painting, and Gainsborough’s Wooded landscape with horse, cart and figures.

Valuing the collection is difficult considering the huge prices that Turner has been attracting recently. Although one of his Venetian views sold in New York last year for almost $36 million (£18 million), that was in oils, which traditionally fetch far higher prices than watercolours. Earlier this month, the Tate acquired for £4.95 million The Blue Rigi, but that example was in a league of its own: it is regarded as one of the finest achievements not only of Turner but of any artist working in watercolour.

Watercolours are too delicate to be displayed permanently. The Courtauld is planning to show the Turners next year, as part of an important exhibition on the artist.

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