The Constitutional Court Friday voted 6:3 to nullify the February 2 election.
The court reasoned that the election violated Article 108 (2) of the Constitution because no candidates stood in 28 constituencies in eight southern provinces, and thus making it impossible to hold the election on the same day nationwide.
The article requires that a general election must be held on the same day throughout the country.
The court ordered the Election Commission to consult with the government on the new election date.
The three minority judges who did not think the election was invalid were Chat Chonlaworn, Udomsak Nitimontree and Chalermpol Ek-uru.