Alarmed by the speed and severity of the recession overtaking Britain, business leaders who have been among Conservative Prime Minister Margaret Thatcher's strongest supporters are now anxiously pressuring her government to relax its radically austere economic policies.

While agreeing with Thatcher's long-term monetarist goals of squeezing down inflation, cutting public spending and reducing the role of government in the economy, the business leaders are expressing growing fears that British industry could collapse before ever benefiting from these policies.

As in the United States, the recession in Britain has hit harder because of the government's credit-squeezing monetary measures to try to reduce inflation. It has been made even worse here, however, by an inflation rate now at 22 percent and still rising, the government's record minimum lending rate of 17 percent, and the inflated value of the British pound.

This has increased costs almost unbearably for British businesses at a time when their exports, domestic sales and profits all are plummeting. Already, there have been widespead plant closings, massive layoffs and growing threats of bankrupticies.

With economic forecasters predicting much worse to come for the next year or two or even longer, business leaders have begun to put pressure on Thatcher to alter her policies. They want her to lower interest rates, put a lid on wages, deflate the pound's value, and block cheap foreign imports.

Thatcher and her chief lieutenants so far have refused to do any of this because it would mean deviating from their bedrock policies of tight money, free competition and minimal government interference.

"Industrialists here think it is time for a little flexibility," said John Rice of the Birmingham Chamber of Commerce in England's industrial Midlands. "They realize the government had to get hold of the economy by the scruff of the neck; they chose monetarism, and they knew the medicine would be hard to take. But as the recession gathers momentum, they don't like the government's apparent inflexibility."

"I doubt whether the government yet appreciates the plight of manufacturing industry and the speed at which it is being destroyed," said leading Midlands industrialist Reg Parkes, chairman of the region's branch of the Confederation of British Industries.

"I believe in monetary policy but this government is operating on a political time scale, not a commercial one," Parkes said in a gloomy confederation business survey. "The government must look again at its strategy.

"If at the end of the day we have no industrial base, where do we go?" Parkes said.

In the industrial belt of Scotland, an economist for the Scottish Concil, the region's economic lobbying group, said the outlook there was "very gloomy." In surveys and meetings of Scottish business executives, he said, "There are loud complaints" about the government's interest rates and the high value of the pound in a deteriorating economy.

"They think there has to be some give in the next few months," Parkes added.

Most of the economic policy changes British businessmem are seeking already have been advocated by labor unions and the opposition Labor Party. But Thatcher has been able to ignore them.

The Labor Party has been demoralized since its defeat by Thatcher's Conservatives a year ago and debilitated by an angry power struggle between its left and right wings. Britain's labor unions, the foundation of the Labor Party, have failed to recover the public support they lost, according to opinion polls, with the crippling strikes at the beginning of 1979 that contributed to Thatcher's election victory a few months later.

"Thatcher's government doesn't have to listen to the trade unions very much," said one well-informed economic analyst here. "The question is will it now listen to the business community and make changes."

The Confederation of British Industries, Britain's most powerful business lobby, is pushing hardest for Thatcher's government to follow the Carter administration and reduce interest rates.

As U.S. interest rates fall below Britain's, investors put more of their money in Britain, despite its deteriorating economy. That increases the value of the British pound already inflated by Britain's growing North Sea oil revenues, which have made it a "petrocurrency." The pound is currently worth $2.35, compared to $2.06 when Thatcher took office a year ago.

As the pound increased in value, British products cost more abroad, and export sales go down. Meanwhile, foreign-made products imported into Britain are cheaper for British consumers, which also decreases the domestic market for British firms.

From electronics manufactures in Scotland to textile mills in the Midlands and auto plants throughout Britain, sales are down and factories are closing. Tens of thousands of jobs have been lost since Thatcher's government took office and unemployment will soon hit its highest level since the depression of the 1930s.

"If the pound keeps going up and interest rates don't start going down," a banker in Edinburgh said, "a lot of businesses in Scotland are going to have their backs to the wall."

"It's silly to have a petrocurrency, " explained Dermont Glynn, economic director of the Confederation of British Industries, "and also have interest rates higher than everyone else."

Many economic analysts and bankers here believe the government is looking for a way to reduce interest rates without appearing to be wavering from its monetarist course.

Other business leaders, such as Scottish industrialist Robert Duthie, chairman of the Scottish Development Agency, want Thatcher to impose a wage freeze to hold down pay raises, which have risen as fast as inflation. "If something isn't done soon," said Duthie, who is trying to attract foreign-owned businesses to Scotland, "we will lose all the competitive advantage we used to have on wages."

"It's crazy," an American banker here, said. "Everybody's getting pay raises of 20 percent of more. That just builds in the inflation. It will never come down if the circle isn't broken."

Although Duthie and others note that Thatcher once said she has not ruled out an emergency wage freeze -- as opposed to institutionalized wage and price controls or guidelines -- she and her Cabinet have gone no further than to repeatedly urge unions to show more restraint in wage bargaining.

Executives in the auto and textile industries, which have suffered most from competition by cheaper imports from Japan, the United States and Third World countries, are pushing for import controls. They also would violate Thatcher's free market philosophy.

The concerns of these business leaders, however, are being transmitted to the government by a growing number of back-bench Conservative members of Parliament who won marginal seats in the Thatcher landslide from constituencies where thousands of jobs are being lost in the auto and textile industries.

It was partly in response to their pressure, which recently became an open parliamentary rebellion, that the Thatcher government completely watered down Britain's U.S.-requested economic sanctions against Iran to allow all existing British trade to continue and even expand in the future through modifications of existing contracts.

"Now that their constituents are getting hurt, there is a hell of a lot of dissatisfaction in the Tory ranks with Thatcher's hard-line economic policy," said one close observer of the government. "If things get too bad and Thatcher doesn't make a U-turn, they might eventually want to change the leadership."