The National Gallery and the Tate today announced one of the most significant bequests of paintings ever made to the nation.

Simon Sainsbury, a member of the supermarket dynasty who died just over a year ago aged 76, left 18 works including masterpieces by Edgar Degas, Paul Gauguin, Claude Monet, Thomas Gainsborough, Francis Bacon and Lucian Freud. The extraordinary gift could be worth a hundred million pounds.

The philanthropist and millionaire had opened discussions with the galleries about works in his collection in the late Nineties.

Gallery: Sainsbury bequest

They will be known as the Hon Simon Sainsbury Bequest and will go on show at the venues next summer.

Martin Wyld, acting director of the National Gallery, said Simon Sainsbury's legacy at the National was "truly remarkable".

Simon, along with his brothers John and Tim, funded the Sainsbury Wing at the gallery and paid close attention to the design and building process. Mr Wyld said Simon was also "a devoted trustee".

"He long intended that major works from his distinguished collection should come to the National Gallery and these outstanding paintings will greatly enrich our Impressionist and post-Impressionist collection."

Tate director Sir Nicholas Serota, who has spoken of the difficulties of acquiring new works at today's prices, said he was extremely grateful so many remarkable ones had been bequeathed.

"This is one of the most important gifts in the history of Tate. The sheer variety of works will enhance many different areas of the collection," he said. "Simon 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."

The five works donated to the National Gallery include Snow Scene at Argenteuil by Monet, which is the largest and most atmospheric of 18 snow scenes the artist painted in the Parisian suburb.

The Tate bequest includes Mr and Mrs Carter - one of Gainsborough's earliest works - plus Colonel Blair And His Family-And An Indian Ayah In An Interior (1789) by Johan Zoffany - an important record of East India Company officials.

Nude In The Bath, one of the earliest of a series of paintings by Pierre Bonnard of his wife bathing, is another gift. Meanwhile-Tate's single work by Balthus (French painter Count Balthasar Klossowski de Rola) is joined by three more.

And the gallery previously had no works comparable in subject or style to its new Francis Bacon, Study For A Portrait (1952).