London: Even as British lawmakers prepared to authorise military action against Islamic State militants in Iraq, Prime Minister David Cameron warned that the anti-IS campaign could take `years`.
Taking part in the debate at the House of Commons, Cameron said the offensive could extend for years and that "we must be prepared for that".
The Islamic State militants "have already declared war on us" and that there isn't "a walk on by option", BBC quoted Cameron as saying.
Cameron said US President Barack Obama has categorically stated that he wants Britain to join the air strikes.
He sought to highlight the fact that Arab countries were part of the coalition against IS, adding the UK must play its role too.
He, however, added that UK ground troops would not be deployed.
Describing IS as a threat to national security, the British PM said the militant group was a terrorist organisation unlike any other the world has seen before.
"This is not a threat on the far side of the world," Cameron was quoted as telling MPs by the BBC.
Muslims must "reclaim their religion from these extremists", he said further.
According to him, a successful campaign would ensure a stable Iraq and Syria.
"This is not 2003 but we must not use past mistakes as an excuse for indifference or inaction."
Leader of the Opposition Ed Miliband said IS is a threat to "anyone who does not subscribe to their deeply perverted ideology".
Their ideology has "nothing to do with the peaceful religion practised by people across the world and by millions of our fellow citizens, who are appalled by what we see", he added.
The lawmakers are expected to vote in favour of joining air strikes against IS militants despite echoes of the unpopular US-led 2003 invasion under Tony Blair.
Cameron had earlier argued that Britain should not be "frozen with fear" over fresh military action in Iraq.
Six British Tornado fighter jets based in Cyprus are poised to begin raids on IS within days or even hours.
If the vote is passed, Britain would join the US and France in launching targeted strikes on the IS group in Iraq, where it controls swathes of territory, as in neighbouring Syria.
IS fighters have beheaded a British aid worker, David Haines and two US journalists, and are holding two other Britons, Alan Henning and John Cantlie.
Britain will not as yet join US-led air strikes on Syria, which are backed by five Arab states. Cameron`s government says a separate parliamentary vote would be needed for that to happen.
Foreign Secretary Philip Hammond said that Britain was prepared to fight the IS group until "they no longer pose a real threat".
"We judge that ISIL (another name for IS) does pose a serious threat to our security and we will continue to deal with ISIL... until such time as we judge that it does not pose such a threat," he told BBC radio.
(With agency inputs)