ritain's opposition Labor Party announced the resignation today of its leader, Michael Foot, the aging pillar of the party's left wing who last week led Labor to its most decisive defeat in 65 years.

Foot, 69, will formally step down at the party's annual conference in October. But the succession struggle was already under way before Foot's decision was disclosed. A number of candidates have made their intentions known, assuring a donnybrook unless the choice can somehow be brokered in advance.

The three front-runners are Neil Kinnock, an engaging 41-year-old Welshman on the party's left; Roy Hattersley, 51, a Labor moderate who is also a popular newspaper columnist, and Peter Shore, 59, who is on the party's right on most issues and has the most experience in government.

Foot's departure was inevitable in the aftermath of the election debacle that left Labor with a substantially reduced contingent in Parliament and saw its popular vote heavily eroded by the centrist Alliance of Liberals and Social Democrats. Although he had a long and impressive career as an ideological firebrand, Foot proved a hapless leader. He was unable to unite his fractious party and was regarded as an improbable prime minister even by many Labor loyalists.

Labor's deputy leader, Denis Healey, 65, let it be known today that he also will step down at the party conference. For Healey, the campaign was a frustrating and ultimately disastrous experience. Most opinion polls showed that he would have been a far stronger candidate against Conservative Prime Minister Margaret Thatcher than Foot.

But the party could not bring itself to jettison Foot. Once Labor's shaky campaign was under way, Healey only added to the troubles by publicly disagreeing with the party manifesto's commitment to a non-nuclear defense policy and, later, accusing Thatcher--in defiance of public opinion--of "glorying in the slaughter" of last year's Falklands war.

With those two gaffes, it became clear that Healey's moment, like Foot's, had passed.

There appear to be two main elements to the upcoming struggle for Labor's mantle. The first will be to find the right person to reconcile ideological divisions. Because there is so much time before the party ballot is held, the candidates will be scrutinized on their adherence to such planks in the platform as unilateral nuclear disarmament, withdrawal from the European Community and large-scale nationalization of industry.

Labor's center of gravity has shifted sharply to the left since the defeat of its last government in 1979. The results this time plainly represent national repudiation of those leftist policies. But, experts say, it may well be that the deciding votes in the party's internal selection process will still be cast by left-wing unions and local constituency groups.

The second element in the choice will be to find the person best equipped to revitalize the party's image. Foot's ramshackle appearance and the considerable derision in the media of the party's campaign style gave Labor the aura of a loser.

Measured against these criteria, most political analysts said today that Kinnock will be the candidate to beat. He is identified with the left and was loyal to Foot. But he has come under attack from those farther to the left. As an ambitious potential party leader, Kinnock privately acknowledges that ideological compromises will be essential in the next few years if more of the party's moderates are not to drift to the Alliance or even the Tories.

In the election, Labor prevailed only in its strongholds in the depressed industrial north, Scotland and the inner cities, which is not enough if it is to claim power again. It received only 28 percent of the vote, its lowest poll since 1918.

Kinnock's greatest asset, however, is his vigor and charm. With a husky voice, a warm Welsh working-class accent and an easy smile, Kinnock comes across especially well on television. He played a prominent media role in the campaign, providing Labor with some of its few upbeat moments.

Critics say Kinnock is intellectually shallow, lacks any experience in government and devotes too much attention to his public personality.

Hattersley is given the second most likely prospect. He has a well-rounded party profile, a background in government and an acceptable rhetorical style. On the nuclear issue and European Community membership, his views are probably too moderate for the party's left. And unlike Kinnock, he does not have strong support in the unions or local party groups, who hold 70 percent of the votes in Labor's "electoral college."

The remaining 30 percent of the votes belong to the members of Parliament, where Hattersley is thought to have the edge.

Shore is probably too old, pundits were saying today, as he would be nearing his mid-sixties by the likely time of the next national contest. Moreover, Shore is still identified with the governing party of the 1960s and '70s at a time when the torch seems to be passing.

There will also be several dark-horse candidates, although none is given much chance at this stage. Some of the names floated for leader--including veteran left-winger Eric Heffer and former Cabinet minister Gerald Kaufman--may end up in the contest for deputy leader.

While party strategists agreed today that it would be desirable to avoid the extensive skirmishing associated with Foot's selection in 1980, Labor's internal tensions remain unresolved. "I don't see how they can avoid a fight," one expert said, "which is exactly what they don't need."