LONDON, AUG. 6 -- David Owen, once a rising political star, stepped down as head of the Social Democratic Party today after the membership rebuffed him and voted to merge with the Liberal Party to create a united third force in British politics.

Owen, a former Cabinet minister and one of the four founders of the Social Democratic Party, said his party was "deeply and predictably split," but he vowed to remain in Parliament and "lead the campaign for social democracy."

Liberal Party leader David Steel, who surprised Owen by calling for a full merger immediately after the two parties' Alliance fared poorly in the June 11 parliamentary election, welcomed the result of the Social Democrats' referendum.

"I am confident that the ballot of the Liberal Party will show a large majority also in favor," Steel said.

The two parties campaigned as the centrist Alliance in the last two elections but were unable to crack the mold of British politics, in which either the Conservative Party or the socialist Labor Party has governed for six decades.

In the June election the Alliance won only 22 seats -- 17 Liberal and five Social Democrats -- to 376 for Prime Minister Margaret Thatcher's Conservatives and 229 for Labor, and Thatcher sailed into her third term in office.

The Alliance suffered from having the dual Steel-Owen leadership and two differing party policies. Steel's party was closer to the Labor doctrine of unilateral nuclear disarmament, while Owen argued that Britain should keep its independent nuclear deterrent force.

Steel said the two parties should merge completely in a "democratic fusion;" Owen said it was a "recipe for disunity." But other Social Democratic leaders saw merger as the only way to have a chance to break the Conservative-Labor monopoly on government.

The Social Democratic Party's 58,000 members were asked to vote by mail, and the results announced this afternoon showed 77.7 percent voted, with 25,897 in favor of merger and 19,228 against.

An hour later Owen, 49, said he was stepping down and recommending that other opponents of merger also step aside while the Social Democratic Party, or SDP, negotiates the terms of unity with the Liberals.

"The members have decided, as they have every right to do, to seek a merger with the Liberals against my advice, and in the circumstances I do not believe I should continue as their leader during the period of negotiations," he said.

But when pressed about his future in a television interview, Owen said, "Don't write me off, or the party, or the SDP, or social democracy."

The Liberals will consider the merger at their conference in mid-September. After negotiations, members of the Social Democratic Party will vote again on whether to approve the constitution of a new, united party. The process is expected to take at least until the end of the year and perhaps months longer.

Steel, 49, said he was saddened by Owen's decision. "I think he is making a mistake," Steel said, describing Owen as "a very considerable politician."

Steele also hinted that he might not stand for leadership of a new merged Alliance.

"I take the view that there is a strong case for someone new who isn't David Steel or isn't David Owen," he said.