Renewed negotiations to end Britain's six-week-old national steel strike broke off in angry disagreement today, renewing fears that the stoppage might grow into a crippling general strike by British unions against the conservative economic policies of Prime Minister Margaret Thatcher.

The strike by the 150,000 workers of the ailing government-owned British Steel Corporation, who are fighting for higher pay and against plant closures that would claim 50,000 jobs, already has spread to the smaller but relatively prosperous private steel industry. Secondary pickets are beginning to block deliveries of stockpiled steel to other industrial firms.

Tonight, Bill Sirs, leader of the Iron and Steel Trades Confederation, threatened the steelworkers might now be joined by those in the mining and transport industries whose jobs will be threatened by the contraction of British Steel.

"If that happens," Sirs warned, "it will worsen considerably, and that is what we wanted to avoid."

Last week leaders of the Trade Union Congress, Britain's umbrella labor union organization, warned government officials of serious social unrest if Thatcher's economic policies were not modified to reduce the pressure for lower pay raises and huge immediate job cuts in nationalized industries.

Industrial workers in Wales staged a one-day strike last week to protest job cuts in the steel and coal industries there and will begin continuous protest actions on March 10. Employes at the government-owned British Leyland automaking conglomerate also are growing restive over deadlocked wage bargaining and the firing of a shop steward who opposed management plans to improve efficiency by closing 13 plants and eliminating 25,000 of British Leyland's 150,000 jobs.

Thatcher's survival-of-the-fittest policies for reviving the British economy depend on keeping pay raises at or below Britain's 17 percent inflation rate, increasing productivity by eliminating unnecessary jobs and changing union work rules, and legislation to eliminate some of liberal legal rights of British unions.

Despite settlements that averted possibly crippling strikes by mine workers and water and sewerage employes, Thatcher now faces the prospect of the kind of labor union backlash that brought down the government of the last Conservative prime minister, Edward Heath, in 1974. Union leaders and the Labor Party opposition have attacked her refusal to intervene in the steel strike with government money to pay for higher pay raises in the nationalized industry.

On the other side, hard-line Conservatives in her Cabinet and in Parliament, pushed by the national Conservative press, are insisting that the government take an even tougher line with the unions and strengthen the pending legislation restricting secondary picketing and the creation of closed shops.

"What the hell has changed?" demanded a big, black headline filling the front page of the mass circulation Daily Express yesterday, comparing the present labor unrest with trucking and lcoal government workers' strikes last winter that inconvenienced many Britons enough to cause the replacement of a Labor government with Thatcher's Conservatives.

Thatcher's employment secretary, James Prior, a relative liberal in her government, argued successfully within the Cabinet and in a meeting with Conservative members of Parliament this week against toughening the government's pending labor legislation. He warned that Thatcher faced the possbility of a general strike if she antagonized the unions further.

"We have to be careful we don't fall into a dangerous situation by mistake," Prior told reporters today. "That's why I've held the line on the labor legislation this week."

It was such a mistake by the management of British Steel that apparently led to today's breakdown -- after only an hour of discussion -- in the steel negotiations. British Steel sent union leaders a new wage proposal that government officials now admit was "ambiguous."

Steelworkers' leaders thought it offered an immediate 13 percent pay raise, with which they could reach a compromise from their demands for 20 percent. But at the bargaining table, they discovered it consisted of 9 percent now and 4 percent more later if negotiations in individual steel plants produced sufficient productivity gains to pay for the raise.

"We are back to square one," said steelworkers' leader Sirs after he stalked angrily out of the negotiations. "We have walked out because we were misled by them. It's a disgusting state of affairs. We came back to the negotiations table to hear a new offer that was supposed to have been made to us. But we are here under false pretenses."