David Cameron has admitted that “there aren’t enough” moderate Syrian fighters to defeat Isis yet and conceded some of the rebels belong to “relatively hardline Islamist groups”.

He insisted however that the Government’s strategy of relying on 70,000 on-the-ground rebel fighters to seize Isis-held territory following Western air strikes was the right way to defeat the Isis, also known as Daesh and rejected the idea that the only way of beating it was by supporting President Assad’s forces.

Faced with criticism over his claim that 70,000 “moderate” fighters existed in Syria, Mr Cameron said it “wasn’t a figure I invented” but was provided by the security services and was a “best estimate”.

He accepted that not all of the rebel fighters were the “sort of people you bump into at Liberal Democrat party conference” but refused a request to publish the names of groups British intelligence services had identified as part of the 70,000 figure.

Doing so would boost President Assad’s chances of winning the civil war in Syria, Mr Cameron said as he insisted a “third way” between Isis and President Assad was achievable for the future of Syria.

Challenged over the 70,000 figure during his appearance before the Liaison Committee – the group of 32 Select Committee chairs – Mr Cameron said: “Are all of these people impeccable democrats who would share the view of democracy that you and I have? No, some of them do belong to Islamist groups and some of them belong to relatively hardline Islamist groups but nonetheless that’s the best estimate of the people that we have potentially to work with.

“The reason for not breaking down in huge amounts of granular detail exactly who they are is simply this: we’d be effectively be giving President Assad a sort of list of the groups of the people and potentially the areas that he should be targeting and that’s not my approach.”

“People want to say there aren’t enough opposition ground troops – I totally agree, they’re not all in the right places, I couldn’t agree more, they’re not all the sort of people you bump into at Liberal Democrat party conference – correct.

British jets prepare for air strikes in Syria Show all 10 1 /10 British jets prepare for air strikes in Syria British jets prepare for air strikes in Syria A Tornado jet takes off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria British jets prepare for air strikes in Syria Pilots and ground crew prepare combat aircraft Panavia Tornados at RAF Marham at RAF Marham, UK Getty British jets prepare for air strikes in Syria A Eurofighter Typhoon jet takes off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria British jets prepare for air strikes in Syria A RAF Tornado arrives at RAF Akrotiri to begin operations in Akrotiri British jets prepare for air strikes in Syria A Tornado jet ahead of taking off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria, the Ministry of Defence has confirmed. The air strikes were carried out within hours of a vote by MPs in the Commons to back extending operations against Isis from neighbouring Iraq British jets prepare for air strikes in Syria Personnel work on a British Tornado after it returned from a mission at RAF Akrotiri in southern Cyprus British jets prepare for air strikes in Syria Two RAF Tornado GR4's, both with remaining weapons ordnance, approach RAF Akrotiri in Cyprus, as they return to the base after carrying out some of the first British bombing runs over Syria British jets prepare for air strikes in Syria A RAF Tornado takes off from RAF Akrotiri, on the Mediterranean island nation of Cyprus British jets prepare for air strikes in Syria A Tornado jet leaving RAF Lossiemouth in Scotland British jets prepare for air strikes in Syria AKA RAF Tornado arrives at RAF Akrotiri to begin operations in Akrotiri, Cyprus. The RAF has sent two further Tornado aircraft and six Typhoons to bolster aircraft now flying sorties to both Iraq and Syria

He added: “I would agree with all those assessments but the point I would make is: Is there a third way between a Daesh-style state and President Assad the butcher remaining in charge of his country?