The National Transitional Council announced the death of General Abdel Fatah Younes late on Thursday.
The NTC’s Mustafa Abdul Jali said Younes’ death came hours after he was arrested by the rebels for questioning on suspicion his family still had ties to Muammar Gaddafi‘s regime.
The rebels have declared three days of mourning for Younes. Two other Libyan opposition colonels were also killed in the attack.
Younes was one of Muammar Gaddafi’s top generals before he defected to the rebel stronghold of Benghazi in February.
Foreign Secretary William Hague said on Wednesday: “We no longer recognise them as the representatives of the Libyan Government and we are inviting the National Transitional Council to appoint a new Libyan diplomatic envoy to take over the Libyan Embassy in London,” Mr Hague said.