Prime Minister Margaret Thatcher's Conservative Party scored well enough in local council elections throughout much of the country yesterday to justify a decision calling a national election in June, senior party officials said today.

Mounting speculation in political circles over Thatcher's plans for an early ballot has reached the point that a decision by her not to call one would amount to a colossal anticlimax. Everything is now in readiness for an election. Parliamentary candidates have been chosen by the parties, government ministers are "clearing their desks," as one source put it, and the political machinery is in place.

"All Thatcher has to do is push the button," said one expert, noting that the almost universal belief is that she will announce next week that an election is to be held in June, almost a year earlier than required.

However, the choice is ultimately Thatcher's and she has said repeatedly that she would not be rushed into making one. The fourth anniversary of her 1979 victory and the local elections, both of which happened this week, were widely portrayed as the final elements in the timetable for a decision. Thatcher is scheduled to meet Sunday with senior political advisers, and word should begin to leak quickly thereafter on her intentions.

The elections for seats on local councils did not amount to a Conservative landslide, but the party did pick up more than 100 seats. In addition, it held on to its majority in the midlands city of Birmingham, considered an important indicator that high unemployment in urban areas is not hurting the Conservatives very badly.

The opposition Labor Party picked up about 60 seats, and the Liberals also did well, gaining about 100. The losers in the balloting were the Liberals' partners in the new moderate alliance, the Social Democrats, who lost about 10 seats, and independents, who lost about 300.

Political analysts said no firm conclusions about the results of a general election could be drawn from these outcomes. But David Butler, a leading political scientist, wrote in The Times of London that the general pattern "will do little to discourage the Conservatives who are pressing for a June election."

The arguments for a June contest are that it is likely to ensure a decisive mandate for Thatcher to continue her conservative economic and social policies through a second term. Postponing the election until the fall or even next spring, most of her advisers now argue, would leave open the chance that a modest economic recovery might end and her political opponents would have more time to prepare to mount a challenge.

It has been clear all along that Thatcher's personal inclination was to see the term through to its natural end in May, 1984. But by not ending the public speculation about an election and allowing preparations to continue for one, Thatcher has, Ian Aitken of The Guardian wrote, "effectively closed most of her options."

Nonetheless, there are factors that could lead Thatcher to surprise the country and delay the contest. Calling an election next week for June 9 would mean abandoning several major pieces of legislation in Parliament. Moreover, Thatcher might have to forgo two foreign trips--her participation at the Williamsburg meeting of allied leaders in late May and the European summit meeting at Stuttgart in early June. Similar scheduling problems would arise for elections June 16 or 23 as well.