Mr Prompong said Mr Abhisit's proposal, particularly for caretaker Prime Minister Yingluck Shinawatra and her cabinet to resign and be replaced by a neutral prime minister and a neutral government, was not backed up by the Constitution and might even be in breach of it.
He said there was no guarantee that a neutral government and a neutral prime minister would not be fraudulent.
"Having a neutral government and a neutral prime minister would be tantamount to freezing the country and staging a silent coup, showing disregard for the Constitution," he said.
"Mr Abhisit's proposal coincides with People's Democratic Reform Committee's call for the prime minister and the cabinet to resign.
"Since Mr Abhisit graduated from abroad and holds British nationality, he should adhere to democratic principles. Instead, he has forgotten democratic principles and forgotten about people who died calling for democracy in the October and Black May incidents.
"His proposal has exposed his true traits. He is afraid of losing in an election and of the people's voice. His mentioning of reforms and offer to take a break from politics are only to make himself look good.
"I would say that Mr Abhsit has made such an offer because he knows he has a problem over his qualifications to run in an election. He is just deceiving the people.
"The party affairs committee of Pheu Thai willl consider Mr Abhisit's proposal on May 6 and will issue a statement afterward."
The Pheu Thai spokesman called for Mr Abhisit and the Democrat Party to review their stance.
Since the Election Commission had tentatively set the date for a new election, the Democrat Party should help find a way of resolving the conflict by contesting the poll, Mr Prompong said.
Mr Prompong said he would send documents showing Mr Abhisit had been dismissed from military service and a copy of the Constitution by mail to Democrat Party advisory chairman Chuan Leekpai and advisory committee member Banyat Bantadtan to re-check Mr Abhisit's qualifications.
He said many other persons including Mr Chuan and Mr Banyat could still return to lead the Democrat Party without Mr Abhisit.
It was time for the Democrat Party to have a new leader, Mr Prompong said.
Concerning Mr Abhisit's proposal, Democrat spokesman Chavanond Intarakomalyasut said Mr Abhisit was expecting a reply from Ms Yingluck in a few days.
If the prime minister was still uncertain about the legality of Mr Abhisit's roadmap, she could make an appointment to meet for discussion with the Democrat leader, the spokesman said.
Ong-art Klampaiboon, a Democrat Party deputy leader, said Mr Abhisit's move was not to put himself at a political advantage. The prime minister should review her stance and join Mr Abhisit to explore a way out for the country.
The prime minister is key to breaking the impasse, he said.