Fugitive former prime minister Thaksin Shinawatra says he was left flabbergasted by the allegation of a "secret deal" for the Pheu Thai Party to form a coalition government with former parties in the government bloc.

Thaksin strongly denied the Pheu Thai move to a new coalition on Twitter on Sunday, after whistleblower Chuvit Kamolvisit posted on social media earlier that day.

Mr Chuvit said a "secret deal" had been reached by Pheu Thai, Bhumjaithai, Palang Pracharath, Democrat and Chartthaipattana parties to form a coalition.

The deal was struck while Pheu Thai was still in the process of thrashing out differences with Move Forward and six other smaller parties in a planned coalition government. Pheu Thai's U-turn would leave Move Forward on the opposition benches, Mr Chuvit said in his social media post.

According to Mr Chuvit, this Pheu Thai-led coalition would be endorsed by over 376 votes at a joint sitting of parliament, with the support of senators, as required by the law.

The "secret deal" included conditions for Thaksin's return home from Dubai, where he lives in political exile, according to Mr Chuvit.

The success of the negotiations had been reported to Thaksin, who was waiting to hear the result in Singapore, according to Mr Chuvit, who said he would rather keep his source of information secret.

He quoted the source as saying that Move Forward was too much of a risk factor and its leader Pita Limjaroenrat's aspirations for the job of prime minister would be shattered.

On his Twitter post, Thaksin said: "I am flabbergasted by Mr Chuvit's mentioning of a secret deal. The allegation was made despite the fact that Paetongtarn 'Ung Ing' Shinawatra, Pheu Thai's prime ministerial candidate, has reiterated support for Mr Pita, the Move Forward Party, to be prime minister.

"I myself have never met with representatives of any party and never received a phone call. I'm totally confused."