The British Museum says it is getting 25 million pounds ($38 million) from John Sainsbury, the former chairman of Britain's eponymous supermarket chain to help pay for a new extension, a museum spokeswoman said on Sunday.
The gift from Sainsbury, 82, is one of the biggest known single donations to the arts in Britain.
It will go towards a new gallery for temporary exhibitions, a conservation center and new storage space at what is Britain's most popular cultural attraction.
"It's incredibly generous," the spokeswoman said. "It's not a donation to benefit us (just) next year, it's for future generations to safeguard the collection for the nation."
The donation comes against a backdrop of big public spending cuts. Funding for museums and galleries is expected to be trimmed by 25 percent in a spending review to be outlined on Oct. 20 by the finance minister, George Osborne.
The government will contribute 22.5 million pounds to the reported 125-million-pound cost of the extension. Other private donors will help pay the bill.
Sainsbury is a long-term patron of the arts and his family funded and gave its name to a new wing at London's National Gallery that opened in 1991.
He was chairman and chief executive of the supermarket chain between 1969 and 1992 and was made a life peer in 1989. He sits as a Conservative peer in the House of Lords.
Established in 1753, the British Museum has one of the world's biggest collections of antiquities, and attracted 5.7 million visitors in 2009-10.
It houses seven million exhibits, including the Rosetta Stone from Egypt and the Elgin Marbles. Greece, which calls them the Parthenon marbles, has long called for the friezes and sculptures from the Acropolis to be returned to Athens.