Joining air strikes against IS in Iraq would be "clearly lawful", he insisted.
Parliament is due to vote on UK involvement in US-led attacks on IS positions in Iraq at about 17:00 BST.
Downing Street has said a small number of British troops
could be sent to Iraq within hours if the Commons backs British military
action, but not in a combat role. The prime minister's official spokesman said they would be used to guide air strikes, in a humanitarian role and, possibly, to train Iraqi and Kurdish peshmerga forces, although this may take place in neighbouring countries.
'Consensus' The government says it would not extend military action in Syria without a Commons vote unless there is an urgent humanitarian need to do so. Downing Street has pointed out that Mr Cameron has pursued a "very deliberate and measured approach" and had been determined "to keep consensus".
The Conservatives, Liberal Democrats and Labour all back action in Iraq, which has been requested by the Iraqi government, with Labour leader Ed Miliband insisting IS "cannot and should not be negotiated with".
The government is expected to win the vote comfortably although some MPs expressed concerns after Mr Cameron said he did not rule out similar action in Syria in the future.
In other developments:
- The widow and daughter of murdered British aid worker David Haines back RAF air strikes
- Speaking in the House of Lords, the Archbishop of Canterbury, Justin Welby, supported action but called for "an ideological and religious" solution rather than a military one.
- Denmark is to contribute seven planes to the international coalition against IS
- The number of Europeans joining Islamist fighters in Syria and Iraq has risen to more than 3,000, the EU's anti-terrorism chief says
Making the case for British action, Mr Cameron said the killing of a British hostage illustrated that the challenge was not "on the far side of the world" and the "brutal, terrorist organisation" was a "clear and proven" threat to UK lives.
"This is not the stuff of fantasy. It is happening in front of us and we need to face up to it," he said.
'Not fantasy' Citing attacks by IS on targets in Europe, and the growing number of foiled terror plots, he said the organisation "already declared war on us and there was no walk on-by option".
David Cameron: Mission against IS in Iraq will take "not just months but years"
Addressing concerns from MPs that the UK could be dragged into a long war, Mr Cameron said the use of combat troops would be "wrong" and conceded air strikes would not "roll back" IS alone but must be part of "comprehensive" political and humanitarian plan.
Referring to previous external interventions in Iraq, he added: "This is not 2003 but we should not use past mistakes as an excuse for inaction or indifference."
Failure to act in Iraq will make the situation even
worse than the already "catastrophic consequences" of IS's advance
there, Ed Miliband warned
"There is no graver decision for our Parliament and our country. But protecting our national interest, security and the values for which we stand is why I will be supporting the motion this afternoon."
Some MPs also questioned the objectives behind the military action.
Conservative backbencher Edward Leigh said airstrikes could be seen as "gesture politics", Labour's Denis Skinner warned of "mission creep" while Green Party MP Caroline Lucas said "killing extremists does not kill their ideas".
The government does not have to seek the approval of MPs to commence military action, but it has become customary to do so since this first happened over the Iraq war in 2003.
Six RAF Tornados in Cyprus have been ready to carry out air strikes for weeks. Their laser-guided bombs and missiles will be loaded and armed soon after Parliament gives the green light.
The Tornados have already been flying reconnaissance missions over Iraq and will have identified potential targets. But some of the obvious ones, like command and control centres, will have already been hit by US war planes which have been launching strikes for the past month.
The focus of their attacks have now switched to Syria itself. So the RAF will also be looking for targets of opportunity - such as IS fighters and vehicles on the move.
The expected intervention of the UK will not be a game-changer. During military intervention against Libya in 2011, the RAF had three times as many war planes involved. But the fight against IS will be more like a marathon than a sprint.
And the longer it goes on, IS will adapt its tactics, and airstrikes will inevitably become less potent.
Downing Street said UK forces could join the bombing of targets in Iraq after the Iraqi minister of foreign affairs wrote to the United Nations seeking international assistance.
The US began a series of air strikes in Iraq last month, and on Monday it began attacks on targets in Syria.
Jets from Saudi Arabia and the United Arab Emirates have joined US forces in the attacks, and the US says more than 40 countries have offered to join the anti-IS coalition.
Footage released by the Pentagon shows a strike on the Jeribe refinery in eastern Syria
The threat was made in a video showing the beheading of British aid worker David Haines earlier this month. Also on Tuesday, IS released a second video showing UK journalist John Cantlie.
Meanwhile, in the US, FBI director James Corney says the bureau has identified the man referred to in the British press as "Jihadi John".
BBC security correspondent Gordon Corera said UK security services were frustrated at the FBI comments.
He said UK authorities "may well" have known the suspect's identity for weeks, but they wanted to gather as much intelligence as possible without "tipping their hand as to what they knew".
The suspected militant - whom the FBI is not naming at present - is thought to have appeared in IS beheading videos.
Speaking at the UN in New York on Wednesday evening, Mr Cameron said countries must stop their citizens travelling to join jihadist groups.
These apparently included Ibrahim Kamara, 19, from Brighton, whose mother Khadijah told the BBC her son had been killed in a US air strike in Syria on Monday.
She said her son, who had apparently joined al-Qaeda affiliate group Jabhat al-Nusra, had been "brainwashed".
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