The opposition coalition -- Democrat, Chat Thai and Mahachon Parties -- Monday handed an "ultimatum" to Prime Minister Thaksin Shinawatra, demanding him to join in constitution amendment and political reform.

They asked the premier to put his signature on a pact of four political parties Tuesday, which will pave the way for amending the constitution, Thai newspaper the Nation reported on Monday.

As part of the political reform and constitution amendment, the coalition plan to ask a "neutral" figure from the King to lead the job.

However, Thaksin ruled out the opposition parties' proposal, saying he had earlier assigned rectors of universities nationwide to study about the constitution amendment and he would not surrender to those who did not play by the rule.

Democrat Party leader Abhisit Vejjajiva said the only way-out of the current crisis is to amend the constitution, adding that the process would pave the way for drafting a new constitution.

He said the amendment would be acceptable if a "neutral" person appointed by the King headed the mission, the report said.

Editor: Yan