Thailand's Constitutional Court has ruled against the largest opposition party over a law on insulting royals.

The court said on Wednesday that The Move Forward Party's pledge to amend the lese majeste law during last year's general election was tantamount to an attempt to overthrow the royal institution. It ordered the party to stop pushing for an amendment.

Following the court ruling, the leaders of the party said they were not attempting to overthrow the monarchy.

The Move Forward Party came in first in the general election. Its pledge to amend the royal defamation law gained young supporters. It has also sought to eliminate the military's influence from Thai politics.

But the Pheu Thai party managed to form a government by joining forces with pro-military parties, making Move Forward an opposition party.

It remains popular in the country, but the court ruling could open the door for conservatives to try and demand the dissolution of the party.