British Prime Minister Margaret Thatcher said today that if the Palestine Liberation Organization refuses to renounce terrorism and recognize Israel's right to exist, alternative negotiating representatives of the Palestinian people must be sought.

Thatcher's statement at a news conference winding up a three-day visit to Israel appeared intended to put distance between the PLO and Britain, which said after terrorist attacks on the Rome and Vienna airports in December that it was reassessing its position toward PLO Chairman Yasser Arafat.

Thatcher was embarrassed in October when two PLO officials reportedly reneged on their promise to sign an agreement renouncing terrorism and recognizing Israel just hours before they were to meet with British Foreign Secretary Geoffrey Howe.

"If they were prepared to genuinely do that and genuinely make it effective, then it seems to me there would be a new situation which would make it possible to view part of the PLO in a different light," Thatcher said.

"Now, if we cannot do that," she added, "we must find other Palestinian representatives who truly represent the Palestinian people. That, I think, is absolutely vital, because there is no point in negotiating with people who don't represent the Palestinian people."

Thatcher suggested that elections in the Israeli-occupied West Bank might provide new leaders who could serve as an alternative to the PLO.

Her remarks were viewed by Israeli officials as significant not only because of a shift of emphasis in British policy toward the PLO, but because Britain will assume the chairmanship of the European Community in July and will be in a position to influence that body's stance toward the Middle East peace process.

Thatcher, whose visit here was the first by a British prime minister since Israel was founded in 1948, met last night with eight Palestinian leaders from the West Bank and Gaza Strip, who presented her with a petition demanding self-determination for the Palestinian people and condemning the April U.S. air raids on Libya.

The Palestinian leaders also reaffirmed their loyalty to the PLO and said they were committed to the establishment of a Palestinian state in the West Bank, which Israel has rejected.

However, at the news conference, Thatcher said that the "ultimate solution" to the Palestinian problem appeared to be a federation between Jordan and the West Bank and Gaza Strip, where 1.3 million Palestinians live.

"We support the right of self-determination for the Palestinian people. We believe that the present proposal of a federation with Jordan would be the one most likely to achieve success and the welcome of the states concerned and among the wider world. You try always when you are working on these matters to go for the situation which will achieve the widest acceptance, because there's no point in working toward anything that will raise other difficulties and other problems," Thatcher said.

When asked whether she was excluding the PLO from peace talks, Thatcher replied, "We have been discussing representatives other than the PLO. Obviously, if not the PLO, who should represent the Palestinian people? . . . There are different views who should represent the Palestinian people, but we must consider an alternative, because we simply must follow all routes."

Thatcher said she suggested to Israeli leaders that elections be held in the West Bank and Gaza Strip to foster a new, moderate leadership of Arabs. Israeli officials have rejected elections because in 1976 balloting, PLO activists won mayoral posts in most West Bank towns.

Thatcher said she would not carry any specific proposals when she meets in three weeks with Jordan's King Hussein.