Fulfilling another of her campaign promises, Prime Minister Margaret Thatcher today unveiled her proposals for curbing the legal power of Britain's labor unions.

Secondary picketing, a common tactic in strikes here, would be outlawed. Unions also would lose the legal protection they now have against being sued for breach of contract if their members strike while a labor contract is still in force.

Closed shops, which are widespread in Britain, would be more difficult to create. Under a detailed statutory "code of practice" that could be used as the basis for court appeals against closed shops, one could be created "only with the wholehearted support of the workers covered."

New legal rights and government financial protection would be given workers who lose their jobs because they refused to join a union out of "deeply held personal convictions," even in closed shops. Only "genuine religious beliefs" are an acceptable legal reason for refusing to join a union in a closed shop here now.

Secret ballots for union elections, rules changes and strike authorizations would be encouraged by government grants to pay the postage for mail ballots.

Thatcher claimed during the recent election campaign that many trade union members decided to support her because they felt they were being forced by undemocratic union procedures to obey the leadership of militants. Britain's economy was plagued by a series of disruptive strikes last winter.

Thatcher's proposals were made in a group of parliamentary working papers on which formal legislative proposals will be based after consultations with union and business leaders. Thatcher's Conservatives have a sufficient majority in Parliament to push through the changes in whatever final form they take.

Spokesmen for several large labor unions quickly criticized the government proposals last night as unnecessary, unworkable and likely to lead to the kind of disastrous confrontation with the unions that followed more sweeping legislative curbs on labor by the last Conservative government in 1971.

That confrontation and a long strike by miners forced then-prime minister Edward Health to call a new national election in 1974, which the Conservatives lost.

The victorious Labor Party then rescinded the laws the unions opposed.

A spokesman for the General and Municipal Workers union, Britain's second-largest labor group, last night characterized Thatcher's proposals as "an unnecessary nterference in the running of trade union affairs."

The right to put up pickets anywhere in a labor dispute in Britain dated back to 1906, he said. Making unions liable for breach of contract claims, the spokesman added, would threaten them with financially crippling court sanctions in most strike situations. Furthermore, he said, the proposed new rules governing closed shops, would merely lead to more litigation.

Len Murray, general secretary of the umbrella Trade Unions Congress, the British equivalent of the AFL-CIO, called the government's proposals "a major challenge" to the rights of workers and unions. "Whatever the government may claim," he said, "these are not limited proposals. We shall try to persuade the government to change course and I hope that employers, who must realize what problems these problems these proposals could cause them, will do some straight talking to the government."

Thatcher's government also faces possible conflict with the unions later this year over the announced demands for wage increases of 20 percent and more to cover fast-rising inflation. Thatcher has warned both unions and employers that there simply will not be enough money in the economy to cover such large raises and that jobs would have to be eliminated instead.