Sir Richard Dannatt
General Sir Richard Dannatt attends a reception at London Guildhall after a Service of Commemoration to mark the end of combat operations in Iraq on October 9, 2009 in London, England. Chris Jackson/Getty Images

The former chief of UK armed forces has urged Prime Minister David Cameron to "think the previously unthinkable" and send UK armed forces to Iraq to halt the progress of Islamic State (Isis).

Sir Richard Dannatt, the former chief of the general staff, said that air strikes had failed to stop the jihadist group, and called for Parliament to debate sending up to 5,000 soldiers to Iraq.

Isis' shock conquest of Ramadi in Iraq and the ancient city of Palmya in Syria last week has called into doubt the capacity of air strikes alone to dent the progress of the jihadist group.

Writing in the Daily Mail, Lord Dannatt said, "In light of this terrifying scenario, how much longer can Britain and the US continue to show such a lack of commitment to defeating Isis militarily? Their default option of air strikes and limited assistance to indigenous forces has failed thus far.

"We have now reached a point when we must think the previously unthinkable and consider that British troops, acting as part of an international coalition, may be required to mount a ground campaign in Iraq and Syria."

"I am no gung-ho general who says 'just send the boys in and don't worry about the body bags,' far from it, but faced with such a lethal and uncompromising enemy as Isis – and with the lack of political and diplomatic solutions at our disposal – we can no longer rule out 'boots on the ground'."

Lord Dannatt accused politicians of shirking the controversial topic during the general election campaign, and called for a full public debate on the issue.

"What I am calling for today is for a public and political debate to begin immediately, so that arguments for and against the deployment of western ground forces can be aired… An opportunity to generate public support for a fully fledged military intervention was lost, so we have got to start talking about and planning for a potential deployment now. Planning is one thing, a decision to commit is another, and could come later."

Dannatt said that there would be challenges, including securing a UN Security Council Resolution in the face of possible vetoes from Russia and China.

The calls come after the architects of the 2007 US surge credited with quelling Sunni/Shi'ite violence in Iraq in 2007 during the US occupation, said that unless the US sends 20,000 troops to the region it risks losing the battle against IS.

UK Tornado strike jets and Reaper drones have taken part in the US-led air campaign against Isis, with Defence Secretary Michael Fallon announcing in March that they would be joined by RAF Sentinel surveillance aircraft.

Lord Dannatt also argued that Syria's President Assad should step down and be given safe haven outside the country. UK forces are currently helping to train fighters from the dwindling Free Syrian Army in Turkey.