Britain will have to put soldiers on the ground in Syria if it wants to defeat Isil, one of the UK's most senior military figures has signalled.
The stark message came as former military leaders and a senior MP warned the RAF will not be able to sustain an effective bombing campaign because of a lack of aircraft.
The Prime Minister is expected to ask MPs to back air strikes in Syria within days but has insisted there will be no British troops on the ground.
Instead, he believes the estimated 70,000-strong armies of moderate
Syrian forces can defeat Islamic State of Iraq and the Levant with the support of Western air power.
However, General Sir Richard Shirreff, the former Nato deputy supreme allied commander Europe, said they would not be strong enough and would need Western forces alongside them if they want to recapture Isil strongholds such as Raqqa.
"It's not something you are going to achieve with 70,000 so-called Syrian moderates," he said.
"To take a city of 350,000 is going to need a massive force. Any fighting in cities soaks up troops in a massive way. It's very heavily attritional, it's bloody and it's a grim business."
Mr Cameron was also warned that any British involvement in air strikes would be "highly marginal" because the ageing fleet of Tornado fighter jets is too small.
Military sources suggested between two and six extra jets are being lined up to join the eight-strong force already carrying out strikes in Iraq.
But Air Vice Marshal Sir John Walker, the former Chief of Defence Intelligence, said 24 aircraft would be needed if Britain was to have an impact. "Can we sustain an effective bombing campaign against IS in Syria with the numbers of Tornado currently available?
"I would say no, we can't, and a lot of RAF people I speak to feel the same way," he added.
He said that although the UK has 72 Tornados only a limited number are available for operations because support units are being wound down as the jet approaches retirement in 2019.
Julian Lewis, Tory chairman of the Commons' defence select committee, said: "The difference the UK can make by joining the bombing effort to the challenge of eliminating IS will be highly marginal."
A Ministry of Defence spokesman said: "We provide around a third of the coalition's high-end precision strike
capability - a figure that will increase if we secure parliamentary approval to strike Isil in Syria as well as Iraq.
"That means we can strike the most difficult targets at speed, with high precision and minimal collateral damage."