Former Democrat MP Wiratana Kalayasiri died of cancer on Friday at the age 69.
The veteran politician from Songkhla had been admitted to Siriraj Hospital for treatment since January this year.
His body will be moved to his home province of Songkhla where funeral rites will be held.
Wiratana made his political debut as a member of Songkhla provincial council before entering national politics and being elected as an MP under the Democrat banner in a 2004 by-election. He also contested the general election in March this year but lost.
As head of the Democrat Party’s legal affairs team, Wiratana was in charge of several high-profile investigations into alleged irregularities and malfeasance.
One of them was a controversial bill sponsored by the Yingluck Shinawatra administration to authorise the Finance Ministry to seek 2 trillion baht in loans for infrastructure development projects. The bill was ruled unconstitutional by the Constitutional Court.
Democrat leader Jurin Laksanawisit offered condolences to his family and credited Wiratana’s legal expertise for helping the party. The Democrats had lost one of their most valuable members, he added.