Libya's Supreme Constitutional Court ruled on Monday that the controversial appointment of the Muslim Brotherhood backed Prime Minister Ahmed Maiteeq was illegal, Al Arabiya reported.
"The ruling decided ... the appointment of Mr Ahmed Maiteeq as premier of the interim government was unconstitutional," state television reported.
The decision is likely to deepen Libya's political crisis at a time when renegade Gen. Khalifa Hafter is waging an offensive against Islamist militias. Hafter has warned he will detain Islamist lawmakers, accusing them of financing militias which he blames for much of Libya's chaos.
Last month, parliament elected Maiteeq in a chaotic vote which had been disputed by some lawmakers and judicial experts. the dispute prompted incumbent Prime Minister Abdullah al-Thinni to refuse to hand over the post until the judiciary decided on the matter.
Libya has sunk into chaos in recent past years following the downfall and the killing of longtime dictator Muammar Qaddafi in the country's 2011 civil war.