Britain's battered union movement suffered another blow today when the country's highest court ruled that a tough employer need not recognize his workers' chosen bargaining agent.

The decision, in sharp variance with 40 years of American labor law, is the latest in an unbroken string of setbacks for the supposedly militant unions here.

The five Law Lords, Britain's supreme court, delivered a unanimous verdict in the emotionally charged Grunwick Film Processing case. Granwick is a small mail order photo developing plant in north London. The Law Lords held that Grunwick's boss, George Ward, was not required to recognize a clerical union because the government had polled only strickers on whether they wanted a union to represent them.

The catch-22 was that Ward refused to let government officials into his plant to poll those going through the picket line. Under the U.S. Wagner Act, no such barrier could be imposed and all workers - strickers and non-strickers - would be balloted.

Grunwick employs only about 400 persons, mostly Indian immigrants, of whom 137 have been on strike for 16 months. It is a company of little economic importance, but the fight for union recognition by the strikers, all of them fired by Ward, has become a cause celebre here.

Other unions have thrown their weight and masses of pickets into the struggle, frequently clashing with serveral thousand police and causing injuries on both sides.

Ward, an Anglo-Indian himself, has become a hero of the right wing here. One of the hard-line rightist groups, the National Association of Freedom, has poured advice and other assistance into his cause. It hailded today's decision as "a triumph for the rule of law over the brute force of trade union bullies."

The Grunwick defeat is another episode in the pattern established under Prime Minister James Callaghan's Labor government. The nation's firemen have been striking since Nov. 14, trying to break the government's "guideline" limiting pay increases to 10 per cent. That walkout now appears doomed.

Callaghan rushed in soldiers to do the firemen's work and no disasters have been recorded. The government has refused to make any pay concessions until next year.

The fire fighters, with no experience in industrial disputes, appealed in desperation last week to top leaders of the Trades Union Congress, the British AFL-CIO. They were turned down and advised to settle on Callaghan's terms.

The firemen are not giving in yet and could last on the picket lines for another few weeks. But no knowledgable source here believes they can now break the 10 per cent line.

In much the same way, Callaghan has turned an apparent defeat by militant coal miners into what looks like a victory. The government recognized it could not induce miners to settle for 10 per cent and therefore offered them bonuses tied to output in addition "productivity" deals.

Leftists leaders in the National Union of Miners convinced the rank and file that this was a dangerous piecework proposal and they voted the offer down. Now the miners are beginning to look at the package differently and they have pressed their leaders to reverse the stand. Last week, the miners' executive board voted 15-9 to do just this.

Perhaps the best measure of the unions' plight is the figure for wages. The weekly after-tax pay for a worker with two children now averages $111. Three years ago - after allowing for inflation - it was $126, a whopping 14 per cent higher.

This staggering drop in real income expresses how Callaghan and Harold Wilson before him have acted to cure a wild inflation that many observers feel was generated by the monetary policies of their Conservative predecessors. In effect, Britain's workers are being made to tighten their belts to compensate for the greatly enlarged money supply of 1972 and 1973.

The government hopes that with one more year of wage restraint it can expand output, spending, employment and real income. Then it would seek re-election.

This chain of events raises serious doubts about the popularly held view here and abroad that "unions run Britain." The belief grew up during the last Tory government when the unions, angered by legislation inhibiting their bargaining power, frustrated government wage restraints. Under Labor, they have cooperated to a remarkable degree.

Ironically, one of the "rewards" the unions were to receive in return was a law faintly resembling the U.S. Wagner Act. It set up an Advisory Arbitration and Conciliation Service, something like the National Labor Relations Board, to poll workers on which unions if any should represent them.

The advisory service went into action in the Grunwick dispute, but was only able to determine the sentiments of the 137 strikers. They voted overwhelmingly for the union. Employer Ward refused to give the service addresses for the 260 who continued to work.