Thailand's main opposition party have announced it will boycott an election in February, deepening uncertainty about the poll and fuelling a campaign to overthrow Prime Minister Yingluck Shinawatra's government.

Ms Yingluck called a snap election on 9 December to try to ease simmering tensions but the movement against her is planning mass rallies across Bangkok on Sunday as part of a "people's coup" to force her and the billionaire Shinawatra family out of politics.

The Democrat Party unanimously agreed during a meeting today that their participation in the election would have legitimised a democratic system it said had been distorted by those in power.

"Thai politics is at a failed stage," party leader Abhisit Vejjajiva, a former prime minister, told reporters in announcing the decision not to run. "The Thai people have lost their faith in the democratic system."

It was not immediately clear whether the Democrats, Thailand's oldest political party, would join a protest movement led by former party heavyweight, Suthep Thaugsuban, which wants to suspend democracy and install an appointed "people's council" to reform the country.

Several party members, Mr Abhisit included,have attended rallies this month.

The boycott adds to concerns that powerful forces allied with the Democrats will seek to block an election that is otherwise likely to return Ms Yingluck's Puea Thai Party to power, and perpetuate the influence of her self-exiled brother, former premier Thaksin Shinawatra.

Puea Thai is the latest incarnation of a political machine controlled by the Shinawatra family, which has won every election since 2001 thanks to policies like easy loans, cheap healthcare and a raft of state subsidies.

Those giveaways won Mr Thaksin the loyalty of millions of rural poor voters but have riled a powerful minority - Bangkok's middle classes, bureaucrats, old-money conservatives and top army generals.

Mr Suthep has asked the much-politicised military to support his movement, but it insists it is neutral and has offered to help ensure the election runs smoothly.