Thailand's main opposition Democrat Party has announced it will boycott an election set for February.
Prime Minister Yingluck Shinawatra called a snap election earlier this month in a bid to end weeks of mass protests.
The opposition-backed protests in Bangkok have caused Thailand's most serious political turmoil since 2010.
Democrat Party leader Abhisit Vejjajiva told a news conference it would not be fielding candidates, saying Thai politics was at a "failed stage" and the The Thai people had "lost their faith in the democratic system."
Mass anti-government protests began in November. Photo: AFP
The head of the Thai army told the Bangkok Post the country's political divisions could "trigger a civil war".
General Prayuth Chan-ocha has proposed a people's assembly made up of civilians, not leaders, to heal the divisions.
He said it should be made up of people from both sides of the political divide - known as the "red shirts", those who support Thaksin Shinawatra, and the "yellow shirts", those who oppose him.
His comments came after a defence council meeting on Friday to discuss the 2 February election.
Defence spokesman Col Thanatip Sawangsaeng said the army "is ready to support the Election Commission in organising the elections when asked".
But a military source has told the BBC that privately the army believes it would be better for the election to be delayed - as sought by the opposition parties.
Ms Yingluck won the last elections in 2011, but protesters say her brother - the controversial ousted former leader Thaksin Shinawatra - remains in charge.
Her Pheu Thai party has a majority in parliament, and draws significant support from Thailand's rural areas. It is seen as well-placed to win February's election.
Protests began nearly a month ago after Thailand's lower house passed a controversial amnesty bill, which critics said could allow Thaksin Shinawatra to return without serving time in jail.
Mr Thaksin is in self-imposed exile after he was overthrown in a military army coup in 2006 and convicted of corruption.
