Tripoli - Libya's Islamist Justice and Construction Party announced it was quitting the government on Tuesday after it failed to win sufficient support for a censure motion in parliament.
The party said it was "withdrawing its ministers from Prime Minister Ali Zeidan's government and holds those in the [General National] Congress who backed his government responsible."
Zeidan was "incapable of taking the country where it needs to go," said the party, which is the political arm of the Muslim Brotherhood, the same pan-Islamic group whose elected president Mohamed Morsi was overthrown in neighbouring Egypt last year.
The party has five ministers in Libya's interim administration - those for oil, electricity, housing, economy and sport.
After three weeks of heated debate, the Islamists and their allies failed to secure the 120 votes they needed to censure the prime minister, whose security failings they say were highlighted by his own brief abduction by militia last year.
Islamists were predominant among 99 MPs who signed a petition of protest earlier in the day, accusing Zeidan of a "crushing failure" in efforts to restore security two and a half years after the overthrow of veteran dictator Muammar Gaddafi.