European defense ministers failed today in their latest, and apparently close to final, attempt to agree on the design and production shares of a new jointly produced fighter aircraft.

After 18 months of negotiations, ministers from the five participating countries -- Britain, West Germany, France, Spain and Italy -- said they had reached the "bare bones" of a longstanding disagreement that centers around conflicting British and French designs.

Similar differences exist over which country or countries in unemployment-high Western Europe will get the bulk of the contracts and jobs involved in the $25 billion, 1,000-aircraft production project.

"If the delay" in coming to a decision "is any longer," British Defense Secretary Michael Heseltine said, "we'll begin to threaten the projected in-service date" around the mid-1990s.

Others emphasize a more immediate political implication. "At a time when we speak about technical cooperation" said West German Defense Minister Manfred Woerner, failure at "the most promising and most challenging" test of such cooperation would cause "some doubt about the great deeds behind our great words."

The two-day meeting -- the second in as many months among the ministers -- ended with a new, and "final" target date of mid-July for their respective national aircraft industries to come up with mutually acceptable plans. But sources said that the problem now was primarily political rather than technological.

The five-nation development of a new single-seat, twin-engine combat aircraft is the largest scale joint European defense production effort ever undertaken. Launched in December 1983, it was envisioned as a way for the countries involved to share development costs, and cut the price of production, for replacements for the aging Jaguars, Tornados and F14s in their air forces.

A new European aircraft also would help them avoid spending their money in the United States, and at the same time conceivably could be so cost-effective that it might attract sales outside the five-nation partnership.