Reclusive multimillionaire J. Paul Getty Jr. has created a $25 million endowment to help Britain's impoverished National Gallery purchase works of art, trustees of the gallery announced today.

Trustees chairman Jacob Rothschild said the gift, which is the first installment of an expected total of more than $62 million, was "probably unparalleled by any one individual in the history of British public collections." It is the largest donation for acquisitions made since the gallery was founded in 1824.

With perhaps intentional irony, the Getty endowment will significantly enhance the gallery's ability to compete in the increasingly high-priced world of art auctions, where it and other British institutions recently have lost out to one of the newest and richest American museums, the Getty Museum in Malibu, Calif.

The reclusive Getty, who makes his home in London, is at odds with the home office of the California-based multibillion dollar family foundation, the J. Paul Getty Trust, run by his younger brother, Gordon. The two are sons of the late J. Paul Getty, founder of Getty Oil.

Following last year's $10.1 billion takeover of Getty Oil by Texaco, family members sought a new division of the wealth left by the family patriarch. According to attorneys here for Paul Getty, recent court decisions added significantly to his fortune and will enable the initial $25 million endowment announced today to be expanded to the full $62 million "pending IRS decisions" in the United States.

Paul Getty, 52, was once known as one of the "beautiful people" of the 1960s and early 1970s, when he was a frequent attraction on the U.S. and British social scenes. Now a widower who rarely leaves his home in the Chelsea section of London, he has become known in recent years as a sort of Santa Claus for local causes big and small. During the recently ended yearlong British miners strike, he was said to have given money to funds for both the strikers and those miners who chose to continue working. He once reportedly hired helicopters and donated money to save five baby seals stranded on an English beach after seeing the story of their plight on a local television show. More recently, he donated $5 million to purchase a country estate to be transformed into a facility for the handicapped.

Getty received the most publicity, however, for his donation last year of $500,000 to help save a masterpiece, the famous Duccio portrayal of the Crucifixion, from being purchased from the Manchester City Art Gallery by an overseas buyer. The buyer was the Getty Museum in California, which had bought the painting at auction for $2.25 million but was denied an export license while the Manchester gallery successfully, thanks to Paul Getty, sought funds that would give it a chance to match the bid.

The Getty Museum has become synonymous in Britain for the archetypal big-spending American collectors who, in the view of many irate art lovers in this country, increasingly have outbid the locals on works of art. Some museum and gallery officials say this is an unfair depiction, since the Getty Museum has pledged that it would stand back, at least temporarily, from purchasing any painting here in which a British institution expressed an interest.

"They have been very tactful and careful," said one National Gallery official today following the announcement of the endowment. "The last thing we want is for this to appear to be a Getty-versus-Getty matter."

But officials readily acknowledged that the endowment will significantly enhance their acquisition budget, which has been substantially slashed by the Conservative government of Prime Minister Margaret Thatcher as part of overall cuts in government funding of the arts.

Income from the initial $25 million Getty grant to the gallery, which houses the nation's collection of Old Masters and is considered to have one of the best High Renaissance painting collections in the world, will come to about $2 million a year.