Opposition parties joined in Parliament tonight to introduce a motion of no confidence that is believed likely to bring down the Labor Party government of Prime Minister James Callaghan and force a national election here by the beginning of May.

Callaghan made a desperate attempt to win more time by proposing first to the House of Commons and then to the nation on television this evening that a crucial parliamentary vote on limited home rule for Scotland, which Callaghan was almost certain to lose, be postponed for several weeks to discuss alternatives.

Angered by Callaghan's refusal to hold the vote immediately and to do everything possible to win Parliament's approval for the plan establishing a legislative assembly in Scotland, the 11 Scottish Nationalist members of Parliament introduced a motion asking Commons to declare that it had no confidence in the government.

After two hours of hurried discussions with the Scottish Nationalists and other minor parties, the Conservative opposition of 281 members of Parliament led by Margaret Thatcher, introduced its own no confidence motion. Then the Liberal Party's 13 members added still another.

The Conservatives' motion, which takes precedence, forces a no confidence debate and vote by the middle of next week. If Callaghan loses that vote, as political observers tonight were beginning to predict, a national election would be held in late April or early May.

Callaghan had been trying to delay the election until October, near the end of the Labor government's full five years in office, because union strife during the winter has left Labor well behind the Conservatives in public opinion polls.

Thatcher, who hopes to become Britain's first woman prime minister, said tonight she thought her chances of winning the no confidence vote were "very reasonable." She may speak on national television later this week in equal time owed the Conservatives for Callaghan's television appearance tonight.

Although much more wooing of the minor party votes is certain to take place over the weekend, it appeared tonight that Thatcher may well have persuaded enough of the three Welsh nationalist and 12 Northern Ireland members of Parliament to join the 305 Conservative, Liberal and Scottish Nationalist members to defeat Callaghan.

The prime minister, who has used a variety of delaying tactics and favors for some of the minority parties to stay in power this long, can count only on the 305 Labor members of Parliament plus a dwindling number of the contested Welsh and Ulster votes.

Callaghan made an apparently unsuccessful attempt to win the votes of three Welsh nationalists today when the government announced the introduction of a bill to compensate Welsh miners suffering from lung disease.

While one of the Welsh nationalists said he might not vote against the government while the bill is being considered, the two others called its timing "totally cynical" and were reported to have been persuaded by Conservative pledges to introduce a similar bill under a new government.

By coincidence, Callaghan's best bargaining chip with Northern Ireland members of Parliament was removed today when the queen's assent was given to the bill passed by Parliament to increase the number of Ulster seats in the House of Commons from 12 to 17 or 18.

Protestant Ulster Unionist members of Parliament demanded last week that if the government wanted to keep their support, it must build an expensive natural gas pipeline to Northern Ireland and give Ulster more home rule. Government officials answered this week that it could not do either.

The evening's frenzy of activity at Westiminster was touched off by Callaghan's announcement that he intended to postpone the vote on the Scottish and Welsh home rule proposals. They were likely to be defeated because Welsh voters overwhelmingly rejected the assembly proposed for Wales and Scottish voters failed to give more than a tiny majority of approval to the companion home-rule plan for Scotland.

Callaghan said he would delay the vote for up to six weeks while the government held discussions with each of the other parties in Parliament to try to reach agreement on a more acceptable plan for giving some measure of home rule to Scotland and Wales.