Democrat Party leader Abhisit Vejjajiva yesterday vowed not to stand for election if all sides accept his proposals for reform.
He said the move was intended to show there was no hidden agenda behind his campaign to broker an end to the political deadlock.
"Let me announce my position clearly: If all sides accept my proposal, I will not run in the next election," Mr Abhisit said.
He said he was taking the stance to demonstrate his determination to support reform, and insisted he had no desire for personal gain.
He did not say whether his party would contest the polls.
The comments earned a rebuke from caretaker Prime Minister Yingluck Shinawatra, who said Mr Abhisit should not attach conditions for others to accept his proposals.
Ms Yingluck said she is still ready to listen to Mr Abhisit's proposals and believes everyone will agree and comply with any ideas that are practical.
But Mr Abhisit questioned the latest move by the caretaker government to organise a general election for July 20, labelling it a desperate attempt to cling to power.
Going to the polls when the conditions are not right will not solve the problems plaguing the country and could potentially lead to bloodshed, Mr Abhisit warned.
"The options offered by the government or the People's Democratic Reform Committee [PDRC] could lead to deadly clashes," he said. "I am of the view that all these options are risky and are not the answer for Thailand."
Tensions heightened on Wednesday after it was announced a new election is tentatively scheduled for July 20, prompting the anti-government PDRC to call a new major rally to counter a planned demonstration by the red-shirt United Front for Democracy against Dictatorship (UDD).
The new date for the general election was agreed on during talks between the Election Commission (EC) and the caretaker government on Wednesday.
The EC will draft a royal election decree, which will require approval by the caretaker cabinet before it is sent to His Majesty the King for endorsement.
PDRC leader Suthep Thaugsuban on Wednesday responded by announcing yet another "last-ditch battle" against the government with a major rally on May 14. He said PDRC would begin warm-up activities on May 5 in a bid to draw more support for the main rally.
Red-shirt leader Jatuporn Prompan countered by calling a new red-shirt rally on May 5.
Mr Abhisit said any new election should not be held until all conditions are right. A poll must be free and fair, acceptable to all parties, and lead to the formation of a new government and a new House of Representatives, he said.
The present political situation was clearly not favourable for that outcome, he added.
The Democrat leader is in the final stages of his campaign to convince all sides to accept his proposal ahead of looming decisions by the National Anti-Corruption Commission and the Constitutional Court which threaten the fate of the caretaker government.
Both ruling are expected around the middle of this month.
Mr Abhisit has remained coy on the details of his plan, preferring to keep it under wraps until accord could be reached. But he has promised to reveal more information within days.
The Democrat leader has not yet met Ms Yingluck or Mr Suthep to discuss his proposal.
Ms Yingluck said yesterday all sides should try to reach a common position where they can start to move forward together, as suggested by Mr Abhisit.
However, she wanted to hear what Mr Abhisit has to say before making any decisions.
Air force chief Prachin Janthong yesterday voiced his support for Mr Abhisit's peace efforts, saying any move to break the political deadlock is praiseworthy, no matter who initiated it.
Meanwhile, election commissioner Somchai Srisuthiyakorn yesterday said the registration of list-MP candidates for the July 20 will be held on May 25-30, while registration for constituency MP candidates will be held on May 31-June 4.
Domestic advance voting will take place on July 6 and overseas advance voting will take place between June 28 and July 11, he said.
Pheu Thai Party secretary-general Phumtham Wechayachai yesterday welcomed the new poll date, saying the fresh poll will help ease political tensions.