The UK has taken steps to expel five Libyan diplomats, Foreign Secretary William Hague has said.
Updating MPs, Mr Hague said the five - which include the military attache - "could pose a threat" to UK security.
David Cameron said the UK was not ruling out providing arms to rebels in "certain circumstances" but no decision had yet been taken.
Shadow foreign secretary Douglas Alexander questioned the legality and "advisability" of such a move.
The rebels are continuing to lose ground to forces loyal to Colonel Gaddafi and are retreating from their former strongholds along the eastern coast of Libya.
Meanwhile an unconfirmed report from Tunisia's official news agency said the Libyan foreign minister, Moussa Koussa, flew to London on Wednesday afternoon.
The Foreign Office said it was unaware of the visit.
Earlier, the prime minister's official spokesman rejected suggestions the UK's stance on the possibility of supplying weapons to them had shifted in recent weeks.