General Lord Richards, who stepped down as chief of the defence staff last year, said a conventional military campaign on the scale of the attacks which toppled Saddam Hussein in 2003 is needed to crush the Islamist extremist group.

Cricitising the US-led coalition’s reliance on airstrikes, he said: “Ultimately you need a land army to achieve the objectives we’ve set ourselves — all air will do is destroy elements of Isis, it won’t achieve our strategic goal

“The only way to defeat Isis is to take back land they are occupying which means a conventional military operation.