LONDON, JAN. 13 -- The birth of Britain's newest political party, due today, was put off, perhaps forever, when Liberal Party members of Parliament flatly rejected the agreement forged by their leader for merger with the Social Democrats.

Liberal leader David Steel and his Social Democratic counterpart, Robert Maclennan, emerged at 4 a.m. from 12 hours of final negotiations to say they had reached agreement on a set of joint policies. But what began as a day of triumph soon turned into low comedy when a press conference scheduled for noon was postponed, and word began to spread that Liberal members were balking at the merger terms.

At 5, Steel appeared in front of reporters to announce an impasse. "We may have tried too much too soon," he said.

Members of the centuries-old Liberal Party, many of whom fall on the left of the political spectrum on social and defense issues, charged that Steel had given away too much to the Social Democrats. They took particular offense at commitments to upgrade Britain's nuclear defense, to limit welfare payments and to add new sales taxes.

The small Social Democratic Party was launched in the early 1980s by a group of former Labor members who objected to the party's left-wing defense stance.