A breach within Britain's political and military leadership has opened up as David Cameron argued the Libyan leader, Muammar Gaddafi, may be a legitimate target while the Chief of the Defence Staff, Sir David Richards, said he was "absolutely not".
The clash fed a growing concern on the third day of the air assault against Gaddafi that the hastily assembled international alliance is struggling to paper over disagreements about its ultimate war aims in Libya, the future role of Nato and the legitimacy of the rebel groups.
There was also cabinet anxiety that the scale of the initial heavy bombardment may strengthen popular support for Gaddafi in Tripoli and be seen in the Middle East as exceeding the UN security council goal of protecting civilians.
Gaddafi's compound was hit by British missiles on Sunday night in an attempt to weaken his command structure as fighting continued across the coastal towns of Libya.
Tripoli faced a third night of bombing with no sign yet that the allies would call an early halt to attacks on the basis that the no-fly zone had been established.
Senior cabinet ministers admitted that "the emotional optics" of cruise missiles raining down on Libya, backed by coalition military briefings, had unwelcome echoes of Iraq.
Downing Street is urgently trying to help organise the rebel forces so they become a more coherent and visible political and military force.
During a long Commons debate, Cameron eventually won cross-party support from sceptical MPs for his actions, but there was widespread disquiet in the Commons about mission creep, and whether the intervention would end in an unstable partition of Libya.
The rift between Cameron and his defence chief arose after the defence secretary, Liam Fox, said on Sunday that an attack on Gaddafi could be a possibility if it did not lead to civilian casualties. When asked whether Gaddafi was a legitimate target, Sir David replied: "Absolutely not. It is not allowed under the UN resolution and it is not something I want to discuss any further."
Cameron told MPs: "The UN resolution is limited in its scope and explicitly does not provide legal authority for action to bring about Gaddafi's removal by military means. We will help fulfil the security council aims, and leave it to the Libyan people to determine their government and their destiny, but our view is clear that there is no decent future for Libya with Colonel Gaddafi remaining in power."
Later Cameron's spokesman argued it was lawful to target Gaddafi if he was seen as organising the threat to Libyan civilians, pointing out the security council's objective is the protection of civilians.
A summary of the legal advice given to the cabinet by the attorney general, Dominic Grieve, was published. It implies attacks on Gaddafi are lawful if he poses a threat.
The head of the US Africa Command, Gen Carter F Ham, said attacking Gaddafi was not part of his mission.
A French spokesman said that even if the Libyan leader's exact location were known, he would not be targeted. However, the French foreign minister, Alain Juppe, said he hoped the allied attacks would topple Gaddafi. "It is very probable that faced with the increased fragility of the regime, it falls apart from within. In any case that is what we are hoping for."
Barack Obama, on tour in South America, echoed the dispute in London, saying there was no contradiction between the Pentagon saying removal of Gaddafi was not a goal and the White House saying it was. The aim of the military was restricted to fulfilling the mandate of the UN, which was to protect the civilian population, but the White House and the state department was working for his removal.
"I have also stated that it is US policy that Gaddafi has to go and we have a wide range of tools to support that policy," Obama said, making it clear he had in mind diplomatic pressure.
Obama said the core principle was when a leader loses legitimacy and turns his army on civilians, the international community cannot respond with "empty words".
Republicans, while backing intervention to prevent a massacre in Benghazi, objected to Obama going to war without proper consultation with Congress and without a clear mission. Although Obama said he had expected to transfer command from the US to Europe within days, that is being held up by a Nato dispute involving Turkey, which objects to the scale of the attack on Libya.
Ham insisted he was not worried about mission creep. Most of the action to destroy Gaddafi's air defence systems and push back his forces from Benghazi had taken place in the first 24 hours and he did not anticipate further action on that scale.
Ham said he also saw coalition partners taking a bigger share in the days ahead. There had been 60 sorties on Sunday in which coalition planes took part in about half, 70 on Monday, and again about half were non-American. New members were joining the coalition, with Canadians and Belgian forces being added and an expectation of more to follow.
A total of 12 Tomahawk missiles had been fired in the last 24 hours, he said, aimed at a Scud missile base, a regional command centre and a repeat attack on an air defence system. US, British, French, Italian and Spanish planes were over Benghazi and he expected the no-fly zone soon to extend along the length of the coastline.
Ham insisted the mission was not to provide air cover for the rebels, only to protect civilians.
But Mark Toner, the state department spokesman, suggested that regime change was an aim after Gaddafi's failure to honour the ceasefire he declared at the end of last week. "What we are trying to do is convince Gaddafi and his regime to step down from power … that remains our ultimate goal."