King Hussein of Jordan has warned PLO chief Yasser Arafat that he will announce his unilateral acceptance of President Reagan's Mideast peace proposals if Mr. Arafat ''does not make up his mind on time,'' according to Jordanian officials and aides to Mr. Arafat.
The Jordanian monarch is further reported to have told Arafat during almost 10 hours of talks earlier this week that he would make his announcement following a plebiscite among Jordanians and Palestinians on the east bank.
Officials describe the King's threat as ''a bargaining chip among leaders.''
Said one Jordanian official: ''Arafat says he has to go back to the representatives of his people, so the King says he too has to ask his people.''
King Hussein, according to diplomatic sources in the Jordanian capital, has ''not budged'' from his demand that Arafat endorse a formula which will allow him to accept President Reagan's call for Palestinian self-rule in the Israeli-occupied territories ''in association'' with Jordan.
''King Hussein is squeezing Arafat hard to respond decisively,'' according to a well-informed Arab diplomat.
Arafat left Amman for Kuwait Tuesday where he has been in continuous meetings with the Palestine Liberation Organization executive committee and the central council of Al Fatah, the largest guerrilla group within the PLO.
Arafat is due to return to Amman shortly for a ''decisive'' round of talks with the Jordanian monarch.
The leaking of King Hussein's warning to Arafat coincided with Jordanian press reports that the King will hold ''a national speech'' on April 10 - the King's first public appearance since his talks with the PLO chief.
A possible indication of the outcome of the Hussein-Arafat talks is the imminent arrival in Amman of Egyptian President Hosni Mubarak's top foreign policy adviser, Osama al-Baz. Mubarak sent him, according to Arab sources, following a message from Reagan.
Analysts believe that Mr. al-Baz's mission may signal preparations for the restarting of negotiations on autonomy for West Bank and Gaza Palestinians.
Earlier a senior aide to Arafat said that the PLO leader had pledged to King Hussein to endorse ''a modified version'' of Mr. Reagan's proposals. The aide refused to spell out what modifications Arafat was demanding.
PLO officials reached by telephone in Kuwait refused to reveal details of the lengthy meetings of the PLO leadership in that Gulf state. Yet several senior Palestinian officials told the Monitor that ''something big is about to happen.''
PLO officials point out that Arafat is obliged to move within the framework of the resolutions of last month's meeting of the Palestine National Council (PNC) - the PLO's highest policymaking body. Radical and moderate Palestinians alike concede that these vaguely phrased resolutions are open to various interpretations.
The council endorsed the Arab call for an independent Palestinian state and declared that the Reagan plan could not serve as the basis for a comprehensive Middle East settlement. PNC members also say the demand that the PLO and only the PLO can represent the Palestinian people permeates the resolutions.
''The literal negative interpretation of these resolutions adopted by the pro-Syrian hardliners will lead to a stalemate,'' said an Arab diplomat. ''But, '' he added, ''a liberal interpretation provides enough scope for compromise.''