The prime ministerial vote will elect a replacement to Somchai Wongsawat, the PPP leader who was barred from politics in the same ruling that ordered the disbanding of his party.

'Disguised coup'

The vote follow months of protests by demonstrators from the royalist People's Alliance for Democracy (PAD), which accused the PPP of being a corrupt front for Thaksin Shinawatra, the exiled former prime minister who was toppled in a coup in 2006.

The protests disbanded after a court ruled against the PPP in a vote fraud case.

Supporters of the government called the verdict a "disguised coup".

The decree by King Bhumibol Adulyadej said the parliamentary session had been approved "because the house speaker said the prime ministership is vacant and one third of the lawmakers have asked for the extra house session".

It added that under the current constitution, which was introduced by the military government that followed ousted Thaksin, the new prime minister must be elected within 30 days.

Coalition fears

Four small coalition parties that were in government with the PPP said they had abandoned the party set up to succeed it - the Puea Thai, meaning "For Thais" - and would back the Democrat Party's Abhisit instead.

But Puea Thai insisted that it could still win enough support to take power and form a government.

Thailand is struggling with the impact of its political turmoil and the global financial slowdown.

The finance minister on Thursday forecast economic growth at two per cent next year, the lowest level since Thailand recovered from the Asian financial crisis.