An Israeli shipping magnate who served in the Royal Navy has given £20 million to the National Maritime Museum, it was announced today.
It is believed to be the largest single donation by an individual to a UK cultural institution.
Monaco-based philanthropist Sammy Ofer, 86, stunned museum employees by revealing the size of the gift during a visit yesterday to the Unesco World Heritage Site in Greenwich, south east London.
National Maritime Museum director Dr Kevin Fewster said: "There was an audible gasp across the room when the donation was announced. This is such an incredible piece of generosity, people were quite dumbstruck by it."
The money will be put towards the creation of a £35 million wing at the museum housing a major exhibitions gallery, an open archive facility and learning centre.
The museum's existing south west wing will be refurbished and remodelled to incorporate a new south entrance linking it with the Royal Observatory, plus a new restaurant, cafe and shop.
Prime Minister Gordon Brown called the donation "a unique gift to the nation".
He said: "This generous donation by Mr Ofer will help to ensure that current and future generations understand better the crucial role played by the sea in our history and the lives of those who depended upon it."
Billionaire Mr Ofer has strong connections with Britain's maritime history, having served in the Royal Navy in the Mediterranean during the Second World War.
His international shipping empire operates a significant part of its fleet from London, with more than 90 vessels flying the Red Ensign.
He said: "I look forward to helping this hugely exciting project and believe it will be a great step forward for the museum."
National Maritime Museum chairman and former chairman of P&O Cruises, Lord Sterling added: "Sammy is a man who has been key to the global maritime industry for many years and who, through his close relations with the London shipping community and the City of London, has formed an admiration and respect for the long maritime history of this country.
"We are profoundly indebted to him for making such a farsighted donation of his own personal wealth and in expressing such confidence in the future of the museum."
Dr Fewster said he was confident the project would further increase museum visitor numbers which reached a peak of 1.7million in 2007.
He added: "It will create a spectacular contemporary environment in which more people can appreciate the wonders of our collections and their stories of human endeavour and discovery."
It is hoped construction can begin at the start of next year, subject to approval, to be completed in time for the 2012 Olympics.
Mr Ofer was ranked 226 in the Forbes 2007 World Billionaires list and is estimated to be worth 3.9 billion dollars.