Deputy Prime Minister Wissanu Krea-ngam insisted on Monday the regime's order appointing a committee to handpick the senators did not have to be published in the Royal Gazette, saying the composition of the panel was kept secret to prevent lobbying.
Senate Speaker Pornpetch Wichitcholchai also played down concerns, saying that the Constitutional Court had rejected a petition seeking its ruling on the legitimacy of the order.
The petition was lodged by the Office of Ombudsman which asked the court to rule on the constitutionality of the Senate selection committee.
Pheu Thai Party spokeswoman Ladawan Wongsriwong yesterday said she and the spokespersons of six other parties in the anti-regime alliance had jointly submitted a petition to the National Council for Peace and Order, which is constitutionally responsible for appointing the senators.
This was the second time they asked the junta to disclose who appointed the 250 senators after the selection process was criticised for being plagued with nepotism, she said.
Among the appointees were Gen Sittsawet Wongsuwon, Gen Prawit's brother, and AVM Chalermchai Krea-ngam, a younger brother of Mr Wissanu.
Ms Ladawan said something unusual happened as, normally, official documents the public are supposed to know the details of are published in the Royal Gazette, categorised by type and named in the format.
"Deputy Prime Minister Wissanu Krea-ngam had kept saying the names of the panellists would be disclosed but that did not happen," Ms Ladawan added.
"It's really depressing Thailand had an unusual general election on March 24 that drew criticism worldwide. Worse still, the order appointing the Senate selection committee was not published in the Royal Gazette. This may result in the nullification of the vote to choose the prime minister. This issue should be urgently deliberated in a House sitting because it's a national problem that may cause serious damage.
"More importantly, effectively all of the senators voted for NCPO leader Prayut Chan-o-cha to be prime minister," she said, adding that this cast doubt over whether the Senate's vote for Gen Prayut was lawful.
However, according to Mr Wissanu, the constitution and the organic law involving the elections of MPs and senators do not require this.
"The NCPO didn't reveal the names because of concerns about lobbying for the posts. There's nothing to hide. If the NCPO doesn't reveal the names of the selection committee, I could. There's nothing complicated," he said.