Surapong Tovichakchaikul, a former foreign minister, has resigned as member of the Pheu Thai Party after being banned from politics for five years.
He submitted the resignation letter to Pol Lt Gen Viroj Pao-in, the acting Pheu Thai leader, on Thursday.
The National Legislative Assembly (NLA) voted on March 29 to retroactively impeach Mr Surapong for reinstating former prime minister Thaksin Shinawatra's passport. As a result, he is banned from politics for five years.
Mr Surapong said he did not want what he would do in the future to be linked to Pheu Thai because the new political party law being considered by the NLA might impose unexpectedly harsh penalties.
He denied having conflicts with anyone in the party.
He said he had informed Thaksin, who agreed with his idea of setting up a coordination centre to monitor the government's performances.
Mr Surapong said the centre, to be launched soon, was intended to reflect public opinions, uphold righteousness and justice, maintain transparency under the rule of law and justice, and monitor corrupt practices in the government and private sector.
The centre would take grievances from members of the public and place emphasis on scrutinising the government's work concerning public interests.
He said he would not go on the same path as Suthep Thaugsuban, chairman of the Muan Maha Prachachon Foundation, whom he alleged had called for the people to side with him and solicited donations from them.
Pol Lt Gen Viroj said the party respected Mr Surapong's decision and hoped he would rejoin Pheu Thai in the future when the situation changed.