BRUSSELS, JUNE 30 -- British Prime Minister Margaret Thatcher, sharply criticizing her fellow European Community leaders as lacking in will to curb overspending, today blocked unanimous agreement on guidelines for strengthening the community's shaky financial structure over the next five years.

The 12-nation community approved its 1987 budget at a two-day summit here and took a first step toward narrowing its large farm deficits.

But those accomplishments were overshadowed by the summit's failure to reach a consensus -- as the community normally does -- on the principal topic of medium-term financial reforms. Changes in financing are considered essential if the community is to achieve its goal of creating a single, unified market by 1992.

Several leaders expressed regret that the meeting fell short of a unanimous decision but emphasized that the community would seek to reach a consensus at its next summit in Copenhagen in December.

Jacques Delors, president of the community's commission or executive body, said that the agreement among 11 members showed that there is now "a true commitment" to enact reforms. Diplomats warned, however, that Britain's willingness to stand alone here indicated that there would be a difficult fight before the package could be approved.

Thatcher refused to give Britain's assent to a document outlining objectives for future spending policies on grounds that it did not go far enough to prevent deficits.

The document calls for increasing the community's revenues by changing the method of calculating each country's contribution and for boosting spending to benefit the organization's poorer members. The community expects a $6 billion deficit this year, of which 70 percent results from farm subsidy programs.

"What I saw was less than a willingness to put constraints on agriculture," Thatcher told a news conference after the summit broke up late this evening.

Britain objected specifically to language that it said would have fixed too high a base for future increases in agricultural spending and to commitments to spend money before it was clear how revenues would be raised.

Britain is the second largest net contributor to the community's budget, after West Germany, but Thatcher objected to the community's overall policies rather than to its treatment of Britain. She called for "legally binding controls" on deficit spending and suggested that her partners had a tendency "to fudge issues."

Britain also stood alone in blocking a proposed increase of $1.2 billion in the community's program of research and development.

French President Francois Mitterrand called the summit a "half success."

The summit's principal accomplishment was an agreement crafted by Mitterrand and West German Chancellor Helmut Kohl to reduce subsidies paid to farmers to compensate them for losses due to currency fluctuations.

West German farmers are the program's main beneficiaries, but Bonn was expected to assuage its powerful agricultural lobby by making up the difference from its national budget.

The summit also agreed to postpone, and thus effectively kill, a proposed tax on edible oils and fats that could have triggered retaliatory trade measures by the United States.