The outgoing prime minister's announcement follows the crushing defeat inflicted by the rival Pheu Thai Party in the 2011 general election on July 3.

"I've decided to resign because I could not lead my party to victory in the election," Mr Abhisit said, adding that Democrat executive members will meet to select a new party leader within 90 days, as the law requires.

Before the election, Mr Abhisit said he would step down as Democrat leader if his party won fewer seats than in the previous general election.

The unofficial election result shows that the Democrats won 159 seats on July 3. In 2007, the party won 164 seats.

Democrat spokesman Buranat Samutarak said the Democrats still have strong confidence in Mr Abhisit but it is not known if he will return as party leader. Despite the election upset the Democrat Party will continue to perform its duties with full responsibility to the people.

Under the party's regulations, since Mr Abhisit has resigned as party leader all 18 members of the executive committee have also lost their party posts.

Mr Abhisit will hold the post of caretaker party leader until the party holds a general assembly to elect a new leader and executive committee within 90 days. No date has been fixed, the spokesman said.