BANGKOK (Reuters) - A Thai minister at the centre of a pending legal complaint seeking the dismissal of Prime Minister Srettha Thavisin resigned on Tuesday, in an effort to insulate the premier from possible repercussions.

A group of 40 senators lodged a complaint to the Constitutional Court last week against Pichit Chuenban, 65, saying his appointment last month as minister to the prime minister's office breached the constitution, as he has a criminal record.

The court was due on Thursday to decide whether or not to accept the case, which could lead to Srettha's suspension.

"Even though I have been vetted and honestly believe that I am qualified by law, this matter is linked to the prime minister," Pichit said in his resignation letter, shared with media by Srettha's office.

He said his resignation would allow the country "to move ahead and not impact the administrative work of the prime minister that needs continuity."

It was not immediately clear whether the resignation would have any impact on the complaint submitted to the court.

Pichit was jailed for six months in 2008 for contempt of court after an alleged attempt to bribe court officials with 2 million baht ($55,000) hidden in a paper grocery bag.

His law licence was suspended for five years by the Lawyers Council of Thailand after the incident. The government has said it carefully vetted Pichit's qualifications and was confident it could defend his appointment before the court.

Pichit becomes the third minister to quit Srettha's cabinet, after his foreign minister and deputy finance minister resigned following a cabinet reshuffle last month.

The senators, whose term has ended, are currently lawmakers in a caretaker capacity pending the selection of a new chamber. They have accused Pichit of lacking integrity and ethical standards to hold a ministerial post.

Government critics say Pichit was appointed due to his close relationship with a client, ex-premier Thaksin Shinawatra, who returned to Thailand last year after 15 years in exile. Thaksin, an ally of Srettha, still wields considerable political influence, despite officially being retired.

The government has insisted Pichit was appointed due to his capabilities.

($1 = 36.3300 baht)

(Reporting by Panu Wongcha-um and Panarat Thepgumpanat; Editing by Martin Petty)