The Democratic Party filed a motion of no confidence in Lord Tuivakano and his government three months ago, but it was voted down by 13-11 yesterday.
"All the members of cabinet including all the nobles in the house they combined together to defeat the vote," Akilisi Pohiva, the leader of Tonga's opposition Democratic Party, told Radio Australia's Pacific Beat program.
Tonga is a constitutional monarchy and only nine of its 30 Legislative Assembly representatives are elected by the people, while the remainder are chosen either by the nobility or the government
The Prime Minister is chosen by the King from the members of Tonga's parliament.
"We have no doubt in our mind that we have the support of the people," said Mr Pohiva, referring to the vote of no confidence and the accusations of corruption made by his party.
"Unfortunately we are the minority in the house and that has been the problem and we still have this problem in the house," he said.
Lord Tuivakano said the vote was unpopular with most people.
Mr Pohiva said his party was preparing to hold a meeting to discuss the possibility of initiating legal proceedings against Lord Tuivakano.
He said the Democratic Party had hoped the vote of no confidence would push the prime minister to stand down, but believed the matter could now only be resolved in court.