In an open letter to Thaksin dated last Monday, Chamlong said Sudarat tried to discredit him by lying about his alleged plan in the mid-1980s to oust then prime minister Prem Tinsulanonda, who was selected by the House rather than a general election. General Prem is now the Privy Council president.

"I never called for Prem's resignation," Chamlong said, adding that he had served Prem as his secretary-general and had visited Prem several times recently.

"Sudarat should learn more history before saying it," he said.

Sudarat once served as secretary-general of the Palang Dharma Party when Chamlong was its leader.

In the letter, Chamlong called on Thaksin to step down before the country faces a worse crisis.

"The longer you stay, the more damage the country has to bear," wrote Chamlong, a leader of the People's Alliance for Democracy (PAD) and the premier's former mentor.

Chamlong stated that Thaksin had failed to clarify allegations made against him.

"Many people believe you haven't quit because you're scared of losing your assets. You don't have to feel that way. If you're innocent, you can defend yourself in any court," he said.

On Sunday, Thaksin alleged Chamlong turned against him because the PM could not meet his many demands. He claimed Chamlong even asked be deputy prime minister after Thai Rak Thai Party won the election in 2001.

But Chamlong said in his letter: "Thaksin himself asked me to assume the post but I refused, so he had to find other people."