Cambodian authorities hope the new status will increase tourism
Thailand's government should not have backed Cambodian efforts to seek world heritage status for a temple built on disputed land, Thai judges have ruled.
The constitutional court said the Thai government should have consulted parliament before backing the bid.
The Preah Vihear Hindu temple, near the Thai-Cambodian border, was awarded Unesco World Heritage status on Monday.
Thailand's opposition said supporting the bid jeopardised Thai claims to disputed land in the border region.
In its ruling, the constitutional court gave its backing to suggestions the government had acted improperly.
"The government must consult and get approval from parliament before signing treaties with foreign countries," a court spokesman told a news conference.
Local media reported political and academic opposition to the government's decision.
Bangkok newspaper The Nation said some academics had called for the Thai foreign minister to resign for signing the agreement with Cambodia, and said a group of senators were considering impeachment proceedings.
However, the decision was welcomed in Cambodia, where crowds took to the streets to celebrate.
Prime Minister Hun Sen said the decision gave "a new pride for the people of Cambodia", the Associated Press reported.
The 11th Century temple and the land around it were awarded to Cambodia by the International Court of Justice in 1962, forcing Thai troops who had occupied the area to withdraw.
Preah Vihear became caught up in Cambodia's civil conflict: it fell to the Khmer Rouge in 1975 and was only handed back in 1998.
Bidding for Unesco World Heritage status was seen as an opportunity to attract money for restoration and increase the number of tourists in the area.