A partnership of Nikko Securities International, a Japanese financial firm, and Trusthouse Forte, a British hotel and catering company, has completed its purchase of the Watergate Hotel for $48 million, a lawyer for Trusthouse said yesterday.

The sale follows a series of Japanese investments in prominent D.C. hotels. Last September, the James-Wama-Jidoshagakuin Co. and an unidentified partner set what is believed to be a record for District hotels when they paid $49.5 million -- about $385,000 per room -- for a 90 percent interest in the Hay-Adams, a prestigious property that overlooks Lafayette Square and the White House.

All Nippon Airways Co. bought the Westin Hotel in December for $100 million, and a group of American and Japanese investors bought the Jefferson Hotel for $28 million in April 1989, real estate sources said.

Nikko and Trusthouse paid about $203,000 per room for the 237-room Watergate, which is expected to undergo major renovations.

The deal reflects the growing propensity of Japanese investors to buy American real estate in partnership with Western companies instead of on their own. It also shows that Japanese interest in local real estate is still alive. Analysts have been predicting a decline in Japanese investment here because of economic conditions in Japan and a general softening of U.S. real estate markets.

The seller of the Watergate Hotel, a pension fund for British coal miners, put the Washington landmark on the market last summer, saying that it was pulling out of U.S. real estate because it saw better investment prospects in other countries. The British Coal Board has yet to reveal whether it has found a buyer for two office buildings, a shopping mall and the land under three co-op apartment buildings at the Watergate complex.

The Oliver Carr Co., a large Washington developer, considered buying the complex but broke off negotiations with the coal board in December. A Carr executive said at the time that title to the property was "a tangled web."

Coal board representatives could not be reached for comment. Daniel M. Singer, a lawyer for Trusthouse, said the hotel sale was a cash deal, with no mortgage.

There is pending litigation connected to the sale. Capital Dominion Properties Inc., a real estate broker, has sued Buckingham Holdings Inc., an agent for the coal board, contending that it is due a commission for bringing Trusthouse to the deal. Laura Hamilton, a lawyer for Buckingham, said Buckingham denies Capital Dominion's allegations.

The Watergate, which overlooks Rock Creek Parkway and the Potomac River near the Kennedy Center, lent its name to the scandal that toppled President Richard M. Nixon when burglars working on behalf of Nixon's re-election committee were arrested for breaking into the Democratic National Committee's offices there in 1972.

The coal board has invested $14 million in Watergate hotel renovations since 1988, and the new ownership plans to spend another $8 million to overhaul equipment such as elevators and air conditioning, according to Donald M. Blake Jr., vice president of Joseph J. Blake and Associates Inc., a real estate appraiser. Trusthouse is trying to "dress it up" so it will appeal to the more upscale segment of the hotel market, he said.

Trusthouse, a British company run by lords, earls and knights, had taken over management of the hotel before the sale was concluded. Trusthouse has a stake in about 450 hotels. It focuses on expensive hotels, such as the Plaza Athenee in New York, and budget lodging, with its Travelodge chain.

The company also is known for its airport and flight catering and its part ownership of Kentucky Fried Chicken. In some other hotel ventures, Trusthouse has invested 10 percent of the funds and its partners have invested 90 percent, Trusthouse vice president and general counsel Thomas Farrell recently told a group of real estate executives.