Britain has revoked the diplomatic immunity of Muammar Gaddafi and his family, William Hague confirmed, as the foreign secretary called for the Libyan leader to step down.

Hague said the UK was "working intensively" to establish how many Britons remained in Libya as final evacuation missions were being planned.

Asked whether the Libyan leader could remain in power, Hague told BBC1's Andrew Marr Show: "We have here a country descending into civil war, with atrocious scenes of killing of protesters and a government actually making war on its own, so of course it is time for Colonel Gaddafi to go. That is the best hope for Libya.

"And last night, I signed a directive revoking his diplomatic immunity in the United Kingdom but also the diplomatic immunity of his sons, his family, his household. So it is very clear where we stand on his status as the head of state."

The Foreign Office believes "very few" Britons remain in the capital, Tripoli, and the second city, Benghazi – where HMS Cumberland returned on Sunday to pick up more evacuees. Upwards of 300 are thought still to be in remote desert oil areas, and further military-based rescue missions are thought to be planned.

"All I can say at the moment is that we are working intensively to establish who is still in Libya and where they are to see how we can assist with getting them out of there. We continue to urge British nationals to leave Libya," Hague said.

He conceded that the FCO had a "very bad day" on Wednesday, when a series of problems saw the first rescue flight stuck on the runway in Britain for many hours. But he paid tribute to staff based at Tripoli airport who did a "fantastic job".

On Saturday the UN security council unanimously voted to refer the brutal repression of the Libyan uprising to the international criminal court. Sanctions also included a freeze on the assets of the Gaddafi family and a global travel ban on the embattled ruler and his close associates.