Defense officials in Britain, West Germany and Italy announced today that they had agreed on plans for a new European fighter aircraft and would go ahead with its development without the participation of France and Spain.

The decision follows 19 months of negotiations among the five nations for a new combat plane due to be in service by the mid-1990s.

The $25-billion project had been billed as the largest scale joint European defense production effort ever undertaken.

A series of meetings among defense ministers this summer was unable to achieve a compromise between separate design models proposed by France and Britain.

This week, armament directors from the five were called together in Turin, Italy, where they failed last night in one last attempt at agreement.

The collapse of the five-nation project is a blow to overall attempts at Western European technical cooperation and also is likely to have an effect on relations between Paris and Bonn.

Although West Germany favored the heavier, British design, Chancellor Helmut Kohl was anxious to build on his close ties with French President Francois Mitterrand.

France had argued that its lighter design for the one-seat, two-engine plane would be more cost and performance effective.

The design decision also had direct bearing on which country would have the biggest production share of the aircraft. France had argued that its overseas marketing capabilities and contacts for the Mirage jet fighter series made a French-built plane more salable.

But the other three nations felt that a plane along the British lines, described here as a "high-tech dogfighter," is more suited to their defense needs.

At the same time, defense ministries argued that a decision needed to be made since the "window of opportunity" for development and production of the aircraft is narrowing and existing forces needed to be replaced.

"We have sought to find a way forward which all five nations can accept," British Defense Secretary Michael Heseltine said here today, "but we have also recognized that decisions had to be reached if we are to achieve an in-service date for the aircraft which meets the requirements of our air forces and maximizes export prospects."

Separate announcements in the three capitals said the door would remain open for France and Spain to change their minds.

While defense officials here put the chances at "50-50" that the Spanish ultimately would agree to participate, French agreement was viewed as unlikely.

The air-to-air combat plane, which weighs 9.75 tons, is a modification of the British proposal that is "actually a little lighter than the Royal Air Force would have liked," a defense official here said.

Although the production division has not yet been decided, work on the design has been parceled out at 38 percent each for Britain and West Germany, with each agreeing to buy 250 planes, and 24 percent for Italy, which wants about 150 planes.

Assuming that a large portion of the production will take place in Britain, British Aerospace Managing Director Sir Raymond Lygo said today that the aircraft would help maintain as many as 30,000 existing jobs.

Officials here acknowledged that Western Europe in the past has had marginal success in selling its combat aircraft in other markets with the exception of the British-built Hawk and Sea Harrier, and the French-built Mirage.

But "overseas sales will be a very major ingredient" in building the aircraft, the defense official here said.

The new planes will likely be in competition for sales with the McDonnell-Douglas F18 and Northrop's F20, both built in the United States.