Strong blasts rocked the Libyan capital tonight,with a missile also hitting Gaddafi's compound as a column of smoke rose skywards and anti-aircraft fire rang out.

A missile totally destroyed an administrative building of Gaddafi's residence in Tripoli only about 50 metres from the tent where he generally meets guests.

The allies said Gaddafi was not a target but the compound was hit because it served a role in the command and control network.

Britain said a Royal Navy submarine joined US forces in targeting the air defences of Muammar Gaddafi for a second night.

A British Trafalgar-class sub stationed in the Mediterranean fired Tomahawk missiles at air defence targets in Libya as part of a co-ordinated action for the second night in succession, said the Ministry of Defence.

But one mission, involving RAF Tornado jets, was aborted after it emerged there were civilians within the target area.

Spokesman Major General John Lorimer said: "For a second time, the UK has launched guided Tomahawk land attack missiles (TLAM) from a Trafalgar-class submarine in the Mediterranean as part of a co-ordinated coalition plan to enforce the resolution. We and our international partners are continuing operations in support of the United Nations Security Council Resolution 1973."

The onslaught continued despite a ceasefire announced by the Libyan authorities at 7pm UK time.

Both the US and the UK said they do not recognise it. Earlier in the day, Gaddafi had threatened "a long war", and his forces launched a fresh assault on rebels in Misrata.

Downing Street said that Gaddafi remained in breach of his obligations under last week's United Nations resolution, which the coalition would continue to enforce. "Everyone will recall that in recent days Colonel Gaddafi declared a ceasefire which was promptly violated," said a spokesman for Number 10. "We said then we would judge him on his actions not his words - and we will do so again."

Shortly after the ceasefire announcement, a number of Tornados were seen taking off from RAF Marham in Norfolk, in apparent attempt to repeat the previous night's 3,000-mile sortie to target Gaddafi's ground forces in the north African state. There were also reports of anti-aircraft fire overnight.

RAF Typhoons are in place in the southern Italian air base of Gioia del Colle, after deploying on Sunday from Coningsby in Lincolnshire to be within easy range of Libya Planes from Arab state Qatar, as well as Belgium, Denmark and Spain, have arrived to join Britain, France, Canada and the US in patrolling the no-fly zone established over Libya thanks to the destruction of Gaddafi's air defences.

British MPs will be given a vote in the House of Commons on Britain's involvement in the Libyan mission, following a debate promised by David Cameron after he secured the UN resolution authorising military action. With all three major party leaders firmly behind the operation, only a handful of MPs are expected to voice opposition, though concerns are likely to be raised about the prospect of Britain eventually being asked to provide ground troops.