British Prime Minister Margaret Thatcher announced today that she is calling a general election for June 9, a move she believes will enhance her Conservative Party's political mandate and, she said, best serves "the national interest."

With nearly a year to go before an election had to be held, Thatcher was persuaded by senior advisers that she should take advantage of her strong standing in public opinion polls and recently improving economic figures to ensure the Conservatives the widest possible margin of victory. "The country is ready for an election," she told a meeting of her Cabinet this morning, "why delay?"

In keeping with traditional protocol, Thatcher drove to Buckingham Palace to seek Queen Elizabeth II's approval of her decision before issuing a formal declaration from the prime minister's office at 10 Downing Street. Parliament is to be dissolved Friday, and a three-week campaign will then get under way for the 650 seats in a just-redistricted and enlarged House of Commons.

Thatcher had favored waiting until next fall or even later for the contest, but weeks of mounting speculation among politicians and in the press about an earlier date created a momentum that the prime minister could not--and would not--stop. Choosing the earliest date that had been mentioned puts an immediate end to the uncertainty, so the choice of June 9 was in the national interest, she said.

The leader of the opposition Labor Party, Michael Foot, said Thatcher had been "pushed, pulled and panicked" into an election out of fear that the longer she waited the less her chances of victory. Spokesmen for the new Alliance of Liberals and Social Democrats similarly sought to depict the prime minister's action as a "cut-and-run" maneuver aimed at limiting the chance of unforeseen political reversals.

The creation of the centrist Alliance since the last election--the Social Democrats were formed largely by disaffected Labor supporters in 1981--does add an unpredictable new element in British politics. The Social Democrats' standing has been weakening lately, but the Liberal Party, and in particular its leader, David Steel, has done well in some polls and parliamentary by-elections. The Alliance's long-shot hope is that it could hold the balance of power in the new Parliament.

Nonetheless, Thatcher goes into the campaign as a heavy favorite who could even lead her party to a landslide if the main trends in national opinion surveys over the past year are sustained. Since Britain's victory over Argentina in last year's Falkland Islands war, Thatcher has basked in a reputation for what Tory banners call "The Resolute Approach."

In recent interviews, Thatcher has made it clear that she may eventually need a third term as prime minister to fulfill her ambitious agenda of economic and social reforms, another reason for renewing her mandate now, when support in the country seems solid.

The overriding issue in the campaign will be the Conservative economic policies, which the government contends are slowly bringing Britain out of its prolonged decline. Inflation is down to its lowest level in years, interest rates are also down and manufacturing output is starting to show recovery from the worst recession in the postwar era. Unemployment, however, remains at more than 3 million, twice as high as when Thatcher won election in 1979, and there is no sign that it will start to go down any time soon.

Labor and the Alliance both favor variations of a reflated economy, increased public spending to create jobs and an incomes policy. A Labor victory would mean an especially dramatic reversal of economic strategy that would at the minimum lead to a period of economic confusion as new measures were adopted.

On foreign and security issues, the Labor Party is committed to a "non-nuclear" defense policy, the unilateral abolition of Britain's nuclear deterrent and the ouster of all U.S. bases. The party would also withdraw from Britain's membership in the European Community. The Alliance position would modify Britain's nuclear strategy but would not abandon it.

Because of the chasm separating Labor and Conservative policies, this election offers Britain one of the most clear-cut choices in its recent history. In her attitude toward the Soviet Union and on domestic economic matters, Thatcher, like Ronald Reagan, is one of the most ideologically conservative politicians to lead a western nation in recent decades. By contrast, Labor's Michael Foot is the most left-wing opposition leader to compete for the prime ministership.

The Alliance's "prime minister-designate" is the leader of the Social Democrats, Roy Jenkins, who is a former senior Labor minister. Because of Jenkins' generally lackluster performance since winning his party's leadership, the Liberals' Steel was chosen to guide the Alliance's political handling of the election.