Thailand's three main opposition parties have decided not to contest a snap election called by Prime Minister Thaksin Shinawatra for 2 April.
Mr Thaksin has faced growing calls for his resignation
The announcement came after Mr Thaksin refused to agree to their proposals for political reform.
Democrat Party leader Abhisit Vejjajiva made the announcement, flanked by Chart Thai and Mahachon party members.
Mr Thaksin dissolved parliament and called the snap poll on Friday, after mounting calls for him to resign.
The opposition parties "will not stand any candidates in the election" scheduled for 2 April, Mr Abhisit told reporters after the three parties held a joint meeting on Monday.
The parties had threatened to boycott the poll unless Mr Thaksin agreed a series of demands.
The prime minister agreed to meet them, but said that he would not enter into a pact committing to constitutional changes - a comment which appears to have been the catalyst for the opposition's announcement.
"The opposition proposal did not receive a positive response from the government. This means the government is not sincere about political reforms," Mr Abhisit is quoted as telling a news conference.
Mr Thaksin responded by saying his Thai Rak Thai party "has done its best".
"What the opposition parties do is up to them," he told reporters.
Anti-government protesters have vowed to continue demonstrations against Mr Thaksin, saying they want nothing less than his resignation.
Under the banner of the People's Alliance for Democracy, an ad-hoc coalition of activist, labour and student groups, they accuse the prime minister of corruption and abuse of power.
Tens of thousands of demonstrators took part in a rally on Sunday night in Bangkok, and many say they are planning a follow-up protest later on Monday.
Analysts said Mr Thaksin's announcement of early elections was designed to quell the growing discontent amid various groups who have called for his resignation.
Only a year ago his Thai Rak Thai party won 375 out of 500 seats in the general election. Before the opposition boycott was announced, he was widely expected to retain power, but with a reduced majority.
Mr Thaksin has faced calls to resign since his family sold its stake in Shin Corp, the company he built up before he entered politics, last month.
The sale, which netted the family members and others $1.9bn, has angered many urban Thais, who complained the family avoided paying tax and passed control of an important national asset to Singaporean investors.
What do you think of the opposition parties' decision to boycott the elections? Will you be voting on 2 April? What issues are of concern to you? Send us your comments and experiences.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.