The art of generosity: the man who gave pounds 7m to Tate

The mystery millionaire who has put a fortune behind the transformation of the Tate Gallery has been identified as an 88-year-old art-loving businessman who still works as a financial adviser.

Sir Edwin Manton has already given pounds 7m towards the pounds 31m redevelopment of the Tate's Millbank home in central London. He has now pledged a further pounds 7m for the project and in addition will be leaving the Tate a recently discovered Constable painting, The Glebe Farm, previously unknown to Constable specialists.

Sir Edwin gave the original pounds 7m five years ago anonymously. It was the biggest single donation ever given to the gallery.

In 1994, he was knighted for "charitable services to the Tate Gallery", but the art world did not make the connection between the knighthood and the donation.

The money given by Sir Edwin is crucial to the redevelopment of the Tate at Millbank as the new Tate Gallery of British Art, which will start this year. It will house a collection charting the history of British art from the Tate's holdings, while international contemporary art is moved to the new Tate Gallery of Modern Art at Bankside on the other side of the Thames.

The Millbank redevelopment partly funded by Sir Edwin will provide a new suite of six exhibition galleries, nine new or refurbished galleries and a second entrance in Atterbury Street with an internal staircase leading up to new upper-storey galleries.

Despite his age, Sir Edwin still acts as senior adviser to the American International Group, the company where he has worked for most of his life.

Born in Essex, he was educated at Shaftesbury Grammar School and London University. He went to New York in 1933 when he was 24 as a casualty underwriter for the American International Corporation. He became president of the corporation in 1942 and chairman in 1969. He retired in 1975 but returned in 1982 as a senior adviser to the reconstituted American International Group.

His wealth derives largely from the 4 million shares he has in the company, which are worth more than pounds 260m.

Nicholas Serota, director of the Tate, confirming Sir Edwin as the gallery's mystery philanthropist, said: "It is a wonderful gift and will allow us to transform the way we show British art here on Millbank.

"Sir Edwin is a marvellous man. He is quiet, shy and very modest and has not wanted his name up in lights, which is why he has not had the recognition."

However, it is likely that the redeveloped Tate will recognise Sir Edwin's contribution in a formal way, perhaps with his name over one of the rooms.

Sir Edwin, who lives in New York, lists his recreations in Who's Who as walking and art. In 1987, he was one of several anonymous donors who helped raise pounds 3m to save Constable's Waterloo Bridge for the nation.

The Glebe Farm, the Constable painting which he owns and which he has bequeathed to the Tate, came to light in the US two years ago.

Two versions are already at the Tate, and a third at the Detroit Institute of Arts.