Colonel Muammar Gaddafi is losing his "grip on reality", a Foreign Office minister said, as rebel forces closed in on the Libyan capital.
The rebels said they had launched their first attacks in Tripoli amid reports overnight of explosions and heavy gunfire in the city.
Gaddafi responded with a message on state television condemning the rebels as "vermin" and claiming that they were being chased from city to city.
However Foreign Office Minister Alistair Burt said it was clear that the forces of the opposition National Transitional Council were building the pressure on Tripoli.
He said RAF fighters operating with Nato had played a "significant" role in the last 24 hours, attacking key regime communications targets.
"There has been very significant gains by the National Transitional Council forces. This is a gradual process of putting the pressure on the capital," he told BBC News.
"Our understanding has always been that there would not be an uprising in the capital until people were fairly sure on the ground that there would be some support available from those outside.
"Clearly there are forces that have been waiting in Tripoli for the opportunity to express their opposition to the regime.
"The forces of the National Transitional Council clearly are much closer to Tripoli than they have ever been. They have turned their attention there in an effort to free the Libyan people from the attacks that have been made upon them by the Gaddafi regime over so many months."
Mr Burt said that attempts had been made by United Nations envoys to open negotiations with the regime but most efforts had been rebuffed.
