Bombing Libya was not in Britain's national interest, defence and security chiefs said today in a further dent on David Cameron's foreign policy legacy as Prime Minister.
Lord Richards, the former chief of the defence staff, said he and MI6 chief John Sawers raised doubts over the Government's plans to intervene in Libya in 2011.
He criticised Mr Cameron for failing to conduct a 'rigorous analysis' of the situation in the National Security Council.
He spoke as a damning report by MPs warned that Mr Cameron's 'ill-conceived' military campaign in Libya had fuelled the migrant crisis and spurred the growth of ISIS.
In a devastating verdict, the Commons foreign affairs committee savaged the former prime minister's judgment in rushing to war in 2011, saying the intervention was based on 'erroneous assumptions'.
The cross-party committee accused Mr Cameron of ignoring military chiefs and a lack of reliable intelligence to pursue an 'opportunistic policy of regime change' in Libya.
And it says he gave little thought to how Libya would fare following the removal of dictator Colonel Gaddafi, setting the scene for the country's descent into chaos.
Lord Richards confirmed Mr Cameron had ignored advice. Asked if he believed the air-strikes were in Britain's national interest, he told the BBC: 'Whether it was in our vital national interest to intervene, I think is a matter for debate.
'I always was doubtful, and one or two of my colleagues, John Sawers, also shared that doubt - the head of SIS.
The report by the influential foreign affairs committee claimed the bungled military action in Libya paved the way for a major new terror training base for ISIS fanatics.
'Various tribes, independent militias and ISIL took advantage of the absence of central government to seize control of portions of Libyan territory,' the report states.
The committee said Mr Cameron's Libyan adventure 'was not informed by accurate intelligence', with ministers underestimating the threat that the country could become an Islamist stronghold.
It concludes: 'By the summer of 2011, the limited intervention to protect civilians had drifted into an opportunist policy of regime change. That policy was not underpinned by a strategy to support and shape post-Gaddafi Libya.
'The result was political and economic collapse, inter-militia and inter-tribal warfare, humanitarian and migrant crises, widespread human rights violations, the spread of Gaddafi regime weapons across the region and the growth of Islamic State in North Africa.
Today's report said Mr Cameron's failings in Libya means Britain now has a 'particular responsibility' to assist the war-ravaged country and help deal with the flood of migrants heading from its shores to Europe.
But it insisted ministers should not deploy troops to the country until it becomes more stable, warning they would become 'an accessible Western target' for IS and other militants.
An international coalition led by Britain and France launched strikes against Gaddafi's forces in March 2011 after the regime threatened to attack the rebel-held city of Benghazi.
Mr Cameron claimed the intervention was necessary to prevent a massacre of civilians, but the new report says that, despite appalling human rights abuses over 40 years, Gaddafi had no record of large-scale attacks on Libyan civilians.
Today's report also criticises Mr Cameron for ordering military action despite the reservations of the then Chief of the Defence Staff Lord Richards.
MPs are scathing about the lack of post-war planning. The report cites unpublished research by the House of Commons Library showing Britain spent £320million bombing Libya, but just £25million on reconstruction.
Mr Cameron did not give evidence to the inquiry, saying he was too busy. A Foreign Office spokesman said the decision to intervene in Libya was an international one, called for by the Arab League and authorised by the UN.