Striking a major blow in an escalating international legal battle, Britain acted today to protect its firms against increasing efforts by the United States to enforce its antitrust laws and commercial regulations against multinational corporations and international cartels.

Prime Minister Margeret Thatcher's government introduced unprecedented legislation, which is virtually assured of approval by Parliament, that would block enforcement here of American court judgements against British firms in certain antitrust cases.

The legislation also would block enforcement here of punitive double-and triple-damage awards by the U.S. courts, a common remedy sought in civil antitrust cases. If a British firm is forced by a U.S. court to pay punitive damages there, if would be able to recover the money in a British court.

Finally, the legislation also would authorize officials here to stop British firms from being compelled by U.S. subpoenas or court orders to supply information and documents sought in U.S. antitrust investigations, or by U.S. regulatory agencies.

The British action comes at a time when the United States is trying to win increased cooperation from foreign nations in an effort to control the operations of multinational corporations. One of the most recent U.S. concerns has been in the banking area.

The legislation applied to attempts by any foreign country to impose its laws and regulations beyond its borders on British firms. But the bill is clearly aimed at Washington in retaliation for what the British government calls the accumulaton of attempts by the United States to impose its own economic and other domestic policies on individuals and companies outside its territorial jurisdiction."

The British have been unhappy that U.S. agencies and courts have claimed jurisdiction over and tried to punish British companies operating outside the United States that are subsidiaries or affiliates of U.S. firms or whose activities anywhere are judged to have an impact on U.S. commerce or its foreign trade.

They also object to U.S. claims that firms with just 25 percent American ownership are subject to U.S. law even if they do no business there, attempts by U.S. regulatory agencies like the Securities and Exchange Commission and Commodities Futures Trading Commission to impose disclosure and other requirements on firms incorporated and doing business here, and attempts to force British subsidiaries of U.S. firms to obey U.S. policy in dealing with the Arab boycott of companies that trade with Israel.

"It is one thing for a firm to be expected to abide by the laws of an overseas country while it is doing business in that country," Thatcher's trade secretary, John Nott, said today. "It is quite another thing to be expected to abide by the laws of that country, to accept the judgements of its courts or the requirements of its authorities when operating elsewhere."

Several British firms are currently defendants in major U.S. civil antitrust cases for price fixing activities outside in United States in the shipping and uranium industries that adversely affected U.S. firms and commerce, but which the British claim violated no laws here. These firms could be ordered by U.S. courts to pay punitive triple damages totalling several billion dollars.

"We have tried to solve this situation quietly" through diplomatic negotiations, "but with little success," Nott said. "So we decided to show a little bit of muscle to defend our companies and our sovereignty."

Although Britain has antimonopoly laws of its own, they differ considerably from much more comprehensive U.S. antitrust laws and allow many kinds of mergers, monopolies, and agreement to set prices and limit competition that would be illegal in the United States. Britain also does not allow the awarding of punitive damages in civil cases, a practice Nott today called "one of the most objectionable of all American legal devices."

Meanwhile, it has become American policy, embodied in law, to pursue aggressively antitrust violators and international cartels that control the supply and price of commodities and services like uranium or shipping no matter where they operate if they adversely affect U.S. commerce or trade.

As Nott noted, this trend has angered other important trading partners besides Britain, including Canada, which is now considering more limited protective legislation of its own. Nott said he expected other countries to give Britain's far-reaching legislation "a warm welcome" and perhaps copy it.

The breach between Britain and the United States on this issue widened shortly after Thatcher's new business-supported Conservative party took office last May.

In June, the Justice Department prosecuted seven shipping companies, including two British firms operating outside the U.S. and their top executives, for price-fixing that harmed American shippers. After the British government intervened diplomatically, Nott acknowledged today, they were allowed to plead no contest and pay sizeable fines.

But then, contrary to British expectations, the companies and executives were sued by 34 American shippers for punitive damages that could total several billion dollars.