The late Simon Sainsbury, a reclusive member of the supermarket dynasty, has bequeathed 18 works of art worth an estimated £100m to the National and Tate galleries.

His bequest is the most generous gift to the nation in recent times.

The National Gallery, where Sir Simon funded the much-admired Sainsbury Wing with his brothers, is to receive five works by Claude-Oscar Monet, Edgar Degas, Paul Gauguin and Henri Rousseau.

A further thirteen paintings have been donated to the Tate, including works by Lucian Freud, Francis Bacon and Thomas Gainsborough.

Mr Sainsbury, who died last year, left the rest of his collection to his partner Stewart Grimshaw.

They celebrated their civil partnership ceremony in 2006 after 40 years together.

A great-grandson of the original founders of the Sainsbury supermarket chain, Simon Sainsbury was born in 1930.

Educated at Eton and Trinity College, Cambridge, he was a talented sportsman, a gifted pianist and a passionate art collector.

He was particularly interested eighteenth- and nineteenth-century architecture, and Impressionist painting and dedicated a great deal of his time and energy to the project management of the Sainsbury wing of the National Gallery in London.

It was opened by Her Majesty the Queen in 1991.

During his life Mr Sainsbury donated money to the British Museum, the Fitzwilliam Museum, the National Theatre, the Royal Opera House, Tate, the VA, and the Wallace Collection, where he was Chairman of Trustees for 20 years until 1997.

Nicholas Serota, Director, Tate, said: “Simon Sainsbury was one of the UK’s most private but generous philanthropists, giving his wealth, time and experience to numerous and varied causes especially in the cultural sector.

“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.

“The sheer variety of works gifted will enhance many different areas of the Tate Collection.”

A display of the works donated by Simon Sainsbury to the National Gallery and Tate Collection will open at Tate Britain in summer 2008.