Kerala High Court invalidated the oaths of 20 BJP councillors for failing to follow the legal format.
Elected representatives are legally required to swear only in the name of God or by solemn affirmation.
The court has ordered all 20 councillors and one panchayat member to retake their oaths within four weeks.
The ruling followed a petition by a CPM councillor challenging the validity of the original oath-taking ceremony.