As of now, Abdullah al-Thinni is the caretaker prime minister, second deputy speaker Salah Makhzoum told a news conference.
Libya's Supreme Court ruled that parliament's election of Prime Minister Ahmed Maiteeq a month ago was "unconstitutional", state television said.
Maiteeq's appointment as head of Libya's interim government followed a chaotic vote and was disputed by some lawmakers and judicial experts.
Outgoing Prime Minister Abdullah al-Thinni, a career army officer who resigned in April, refused to hand over power to Maiteeq, who was elected by independent and Muslim Brother Islamists.
Thinni said parliament had given him conflicting reports on the validity of the election and said he would wait for a court decision.
Libya's parliament is divided between Islamists and more moderate forces as well as along tribal and regional lines.
"The ruling decided ... the appointment of Mr Ahmed Maiteeq as premier of the interim government was unconstitutional," television reported.
OPEC producer Libya is struggling with chaos as government and parliament struggle to impose authority on a country awash with arms and militias who ousted former leader Muammar Gaddafi in 2011 but now defy state authority.
REUTERS