The Democrats will ask two state agencies to look into the propriety of Parliament president Somsak Kiatsuranont's nine-day "study trip" to Europe.
The junket with 39 aides, media members and academics said to be mainly sympathetic to the governing Pheu Thai party is costing taxpayers 7 million baht.
The high point of the trip for many of the participants, apparently, will be Sunday's Premier League match between Liverpool and Manchester United, though Mr Somsak has said he would pay for that outing.
Sathit Pitutacha, a member of the Democrat Party's executive board, said on Saturday that he would file petitions with the Office of the Ombudsman and the National Anti-Corruption Commission (NACC) asking them to investigate Mr Somsak.
Parliament president Somsak Kiatsuranont (Photo by Chanat Katanyu)
Mr Sathit said some of the media members invited on the trip were not parliament journalists. They were pro-government journalists, columnists and academics who had been attacking the Democrats, he added.
He said the opposition had earlier asked the House committee on ethics to probe Mr Somsak for allegedly failing to perform his duties impartially. Thus, the party demanded him to resign.
The trip started last Wednesday and is scheduled to end this coming Thursday. Mr Somsak said on Thursday that he would take media groups to study the operations of the BBC, The Economist, the UK parliament and Oxford University.
The itinerary covers England, France and Belgium.
He has insisted that he wants the press to learn from the foreign experience, but admits he is looking forward to watching a football match too.
Mr Somsak also acknowledged some mistakes may have arisen in the participants' name lists, as some of those who were invited were found not to be associated with media companies.
He said he was ready to answer all inquiries.