Tony Blair said Britain must be prepared to invade Iraq again in order to defeat Isis if it is "absolutely necessary".
The former prime minister warned air strikes might not enough to combat the militants, also known as the Islamic State, and ground troops may be the best option to defeat the "fanatical" group.
Blair, who has been the United Nations' (UN) Middle East peace envoy since 2007, wrote a lengthy comment piece on his Faith Foundation website to argue the justification of using ground forces.
Britain has yet to decide whether it will join France and the US in launching air strikes in Iraq and possibly Syria in the battle against the militant group.
Blair said: "We have to fight groups like Isis. There can be an abundance of diplomacy, all necessary relief of humanitarian suffering, every conceivable statement of condemnation which we can muster, but unless they're accompanied by physical combat, we will mitigate the problem but not overcome it.
"Air power is a major component of this to be sure, especially with the new weapons available to us. But - and this is the hard truth - airpower alone will not suffice. They can be hemmed in, harried and to a degree contained by air power. But they can't be defeated by it."
Blair, who ordered the controversial 2003 invasion of Iraq, admitted there is "no appetite for ground engagement" but it should not be ruled out in the future if it is "absolutely necessary".
He added: "Provided that there is the consent of the population directly threatened and with the broadest achievable alliance, we have, on occasions, to play our part.
"To those who say that after the campaigns in Afghanistan and Iraq, we have no stomach for such a commitment, I would reply the difficulties we encountered there, are in part intrinsic to the nature of the battle being waged. And our capacity and capability to wage the battle effectively are second to none in part because of our experience there."
David Cameron is set to attend a meeting of the UN general assembly in New York to discuss the possibility of using air strikes against Isis fighters.
The prime minister previously ruled out sending ground troops into Iraq, but will give a speech at the assembly to call for an international coalition to combat the militant group.