NACC deputy secretary-general Worawit Sukboon said the agency would first send notification to 12 ex-MPs explaining that they must submit the asset declaration or face legal action.
The agency had earlier checked the assets and liabilities of the 12 people, but found nothing suspicious.
Two of the 12 ex-MPs have died since leaving office. If an MP dies before the one-year asset-declaration requirement, the executors of their estate do not have to declare their assets. If they die after one year, their offspring must submit their asset declaration.
The former MPs in question include those from the Democrat, Pheu Thai and Chat Pattana parties.