David Hockney is to give his largest painting to the nation to fulfil what he regards as his patriotic duty.

The 40ft x 15ft picture, Bigger Trees Near Warter, consists of 50 separate canvases. It will hang in the Tate Britain gallery in London.

“The Tate had been asking me for some time for a work of art,” Hockney said. “I felt a duty and as an Englishman I wanted to give something to Tate Britain.”

Asked why he had not sold the painting, Hockney said: “It is obviously worth a lot and I could easily sell it, but these days I’m not that interested in money.”

As each section was finished, an assistant photographed it so that Hockney could study on a computer screen how the pieces would fit together and make any necessary adjustments. The whole was designed to fill the end wall of the Royal Academy’s biggest gallery, where it went on show last year.

The whole picture is of a copse beside a country road in the East Riding of Yorkshire, where the 70-year-old artist now lives after decades in America.

According to next month’s edition of The Art Newspaper, the value of the work is not yet being disclosed. Several Hockneys have sold recently for between £2m and £3m.

The Tate will make a formal announcement early next month. The painting will go on display with large photographs, one of each half of the painting, hung beside it to show that it can also be appreciated as two separate works.

The Tate already owns 108 Hockneys, either bought or given by other donors. Most are drawings and prints; its seven paintings include two of his best-known works, A Bigger Splash and Mr and Mrs Clark and Percy. The latter shows the fashion designers Ossie Clark and his wife, Celia Birtwell, with a cat.

Three years ago Sir Nicholas Serota, the director of the Tate, announced that 23 artists, almost all British, had agreed to donate at least one work to the Tate. Only eight – including Damien Hirst, Antony Gormley, Louise Bourgeois and Sir Anthony Caro – have yet done so.

Serota called for the gifts because his galleries have only a limited amount of money available to buy works. Last year the Tate spent £7.4m on purchases, of which half came from charitable funds connected to the galleries and £2.1m came from income generated by the galleries’ shops and restaurants.

“There are two galleries in the world I have an enormous affinity for,” Hockney said: “the Los Angeles County Museum, and I’ve given it a work, and the Tate.

“America is already enriched with gifts but that is mainly because tax breaks make it easier for living people to give. It’s harder here.”

Last year the Tate was bequeathed 13 works by Simon Sainsbury, of the supermarket family. They included paintings by Francis Bacon, Lucian Freud, Thomas Gainsborough and Pierre Bonnard.

The bequest also included paintings by Monet, Degas and Gauguin for the National Gallery in London.

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