Democrat Party leader Abhisit Vejjajiva said the claim was a “political tale” aimed at convincing the public that there was a movement to bring down the government.
With majority control of the House, the government was not under threat, he said.
The government should avoid stirring up social conflict that could lead to street violence, he added.
Democrat spokesman Chavanont Intarakomalsut said the story lacked “truth and substance” and would only lead to undermining the country’s economic and political stability.
He also threatened to take legal action against the government figures making the claim.
Pheu Thai spokesman Prompong Nopparit and deputy government spokesman Anusorn Iamsa-ard, who is also from the ruling party, earlier had said four groups from business, academia, the military and mass media were planning to topple the Pheu Thai-led government.
Deputy Prime Minister Yuthasak Sasiprapha said he knew nothing of the claim and he had received no reports about it.
Prime Minister Yingluck Shinawatra said she treated such an issue as a political matter.