The Royal Navy has assisted evacuation of around 100 British nationals from Libya.
HMS Enterprise was earlier moved into position off the coast of North Africa before sailing into Tripoli.
The ship was moored offshore as her survey boat, Spitfire, collected people from the Port of Tripoli.
A detachment of armed personnel, usually Royal Marines, are providing force protection to the ship in the event of attack.
The routine follows a similar scenario in 2011 during the uprising when HMS Cumberland, a Type 22 frigate, evacuated foreign nationals and refugees from the civil war.
Many of the consular staff were evacuated last Monday but the ambassador and core staff remained.
Defence Secretary Michael Fallon said: “The Royal Navy is helping British citizens leave Libya based upon Foreign and Commonwealth Office advice.
“I thank the crew of HMS Enterprise for their support and professionalism in carrying out this important task.”
Although the United States used F-16 fighter jets for air cover when their citizens left Libya by road for Tunisia last week, it is not thought any British aircraft will be involved in this mission.
Britain is one of the last countries to wind down its diplomatic mission in Libya following a severe deterioration in the security situation as rebel groups continue fighting each other.
France and America, two of the other principal players in the 2011 war, closed their embassies last week.
The Ministry of Defence said: “As the Foreign Office has made clear, the UK Government will provide assisted departure for a number of UK nationals before suspending consular operations on Monday.
“For operational reasons we will not discuss further details including whether, and in what ways, the MoD could support these efforts.”