The National Anti-Corruption Commission (NACC) has decided Deputy Agriculture and Cooperatives Minister Mananya Thaiset must declare her assets and liabilities despite her insistence she is under no obligation to do so.
NACC chairman Watcharapol Prasarnrajkit said on Thursday the deputy minister must submit her wealth declaration under the NACC law.
Ms Mananya is best known for spearheading a mission to ban toxic farm chemicals.
She earlier contended that, because she became deputy minister within 30 days of relinquishing her post as Uthai Thani mayor, she was exempt from filing the declaration.
However, Pol Gen Watcharapol said the 30-day stipulation only applies to a shift from one cabinet post to another, not from the post of municipal mayor.
The NACC chief said Ms Mananya's misunderstanding stemmed from a misinterpretation of the law governing asset declarations.
Ms Mananya is free to ask to the commission to extend the submission deadline, he added.
Also on Thursday, the NACC chairman said MP Kathathep Techadetruangkul will be invited to explain his asset declaration of lek lai rare-metal objects, which some believe harbour magical powers.
Mr Kathathep is the lone MP of the micro party Palang Thai Rak Thai.
The invitation came after activist Srisuwan Janya demanded a probe into Mr Kathathep's possession of a lek lai piece, believed to be gallium, which the MP declared was worth at least 700 million baht. Mr Kathathep also declared ownership of a maha lek lai ("super" lek lai) valued at 300 million baht.
Experts have warned that appraising items based on their supposed magical value could open a loophole to launder money.
Mr Kathathep earlier said he welcomed an inquiry.