Kuwait's constitutional court ruled that a parliamentary election held in February was void and decided to restore the previous assembly, according to state media reports.
KUNA state news agency said on Wednesday the court ruled that "the decree calling for the National Assembly 2012 election is void, and [ordered] the return of the dissolved council".
Mussallam al-Barrak, a leading opposition MP, described the verdict as "a coup against the constitution" and called for the opposition to take a united stand.
Rulings by the Gulf state's highest court are final and cannot be challenged.
Kuwait Emir Sheikh Sabah al-Ahmad Al Sabah issued a decree in early December dissolving the parliament following youth-led street protests calling for reforms and for the sacking of Sheikh Nasser Mohammad al-Ahmad Al Sabah, former prime minister.
A few days later, the emir issued another decree inviting Kuwaitis to elect a new parliament on February 2.
The court ruled that the second decree was "unconstitutional", thus nullifying the results of the general elections in which the opposition scored an impressive victory.
The previous parliament was controlled by a pro-government majority.