Britain will have to put soldiers on the ground in Syria if it wants to defeat Daesh even if David Cameron wins a parliamentary vote for airstrikes, 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 Telegraph reported.

The Prime Minister is expected to ask MPs to back airstrikes 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 Daesh with the support of western airpower.

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 if they want to recapture the terrorist group’s 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."

Cameron was also warned that any British involvement in airstrikes would be “highly marginal” because the ageing fleet of Tornado fighter jets is too small to carry out an effective campaign.

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.

Sources: MENA