Roy Jenkins, leader of Britain's centrist Social Democratic Party, announced today that he is stepping down and urged party members to quickly choose his 44-year-old deputy, David Owen, to succeed him.

Owen, who was a Labor Party foreign minister while still in his thirties, is regarded as a certain winner for the post. Once considered too abrasive to appeal to a broad British electorate, he won widespread praise for his effective campaigning on behalf of the Social Democrats and their Liberal partners in the moderate Alliance in last week's general election.

The Social Democrats' representation in Parliament dropped from 29 to six in the balloting, but as part of the Alliance they did well enough in the popular vote to make the party leadership a position worth having. Owen and Jenkins were two of the four prominent Labor Party politicians who founded the party in 1981, out of frustration at Labor's swing to the left.

Jenkins, 62, was elected party leader last June in a contest against Owen. Jenkins' seniority and broad experience made him better qualified as a potential prime minister, the membership believed. But Owen soon emerged as a stronger spokesman for the Social Democrats in Parliament and on the hustings.

The departure of Jenkins, following yesterday's announcement that Labor leader Michael Foot also would step down, represents an important shift of generations in British politics. The main contenders for the Labor post are a good deal younger than Foot. And, as is the case with Owen, that relative youth is regarded by their political backers as a crucial asset.

The Liberal leader, David Steel, is 43 and conceded the title of "prime minister-designate" in any Alliance government to Jenkins during the campaign. Since the next election is now at least four years off, that honorary leadership position will disappear, permitting Owen and Steel to act as equals.

Relations between the two men were thought to be strained in the past, in part by their differing views on how close the party partnership should be. But with the Social Democrats' representation in Parliament now only about a third the size of the Liberals' 17 members, the Social Democrats have little choice but to maintain the Alliance relationship as a means of keeping the party profile high.

The Alliance received more than 25 percent of the popular vote in the general election but only 3 percent of the parliamentary seats because of Britain's system of apportioning seats under which only winning in a constituency matters. Although its representation in the House of Commons will be far smaller than the ruling Conservatives and the opposition Labor Party, its influence in the country is still likely to be considerable.

Labor now faces a potentially bruising leadership fight to replace Foot with several candidates already in the race. This could exacerbate the serious differences on policy that were so costly to the party in the campaign. Neil Kinnock, a 41-year-old Welshman on the party's left, is the early favorite to succeed Foot, although Roy Hattersley, a 50-year-old moderate, is also attracting support.

By contrast, the transition among the Social Democrats is expected to be smooth. Jenkins' appeal to the party's 65,000 members to back Owen appears to rule out any opponent. Only members of Parliament are eligible for the leadership jobs.

Of the "Gang of Four" who founded the Social Democratic Party, Shirley Williams and William Rodgers lost their seats in Parliament last week.

Despite the Consevative landslide, the Social Democrats scored reasonably well by gathering a substantial number of second-place finishes in local constituencies. This gives the party a chance to improve its position in the coming years at the possible expense of a faltering Labor Party.

Jenkins had a rough time as party leader. Old Labor colleagues in Parliament attempted to shout him down whenever he rose to speak. Then, as "prime minister-designate" during the campaign, he eventually took a back seat to Steel, who passed him in popularity in the polls.

Owen's reputation for arrogance began to recede during last year's Falklands war, when he won respect for his eloquence in parliamentary debates on the conflict. After unsuccessfully challenging Jenkins for the leadership, little was heard from Owen for a time. He complained to friends that Jenkins was effectively merging the new party with the older Liberals, depriving it of its image as a new force in British politics.