Tony Blair said Britain needed to take action in Iraq and Syria - or face terror attacks in at home.
The former Prime Minister said the UK needed to intervene to stop a 'total disaster'. He insisted that he was not calling for troops on the ground - but suggested the 'selective use of air power' was one option on the table.
Mr Blair said: 'If we don't deal with the Syria issue then the problems are not just going to be for Syria and for the region, the problems are actually going to come back and they are going to hit us very directly even in our own country.'
He added: 'If you talk to security services in France and Germany and the UK, they will tell you their biggest single worry today returning jihadists fighters - our own citizens by the way - from Syria.
'We have to look at Syria, and Iraq and the region in context. We have to understand what's going on there and engage.'
He said that didn't mean 'ground troops' but it we shouldn't 'wash our hands of it and walk away'.
Mr Blair's remarks today come as extremist fighters from the al-Qaida-inspired 'Islamic State of Iraq and the Levant' bear down on Baghdad.
Speaking on the BBC's Andrew Marr show Mr Blair said an ISIS victory would be a 'total disaster and it mustn't be allowed to happen'.
He said: 'We are going to have to engage with it and if we don't then the consequences will come back on us.'
Speaking to the Murnaghan show on Sky later, he added: 'The people who are dealing with are going to pull us into this whether we like it or not.
'We are going to have to take an active role in Syria or Iraq in shaping events.'
He said Britain should 'support' the United States if they take military action in Iraq. 'It's in our interest to stop these jihadists.'
'They are prepared to fight us and they will if they are not stopped,' he added.
But Mr Blair's intervention was slammed by critics of his 2003 decision to take Britain to war against Iraq.
The former Deputy Prime Minister Lord Prescott said: 'He says he's disappointed with what has happened in Iraq, it wasn't as he thought it might happen, but he wants to invade somewhere else now.'
He accused Mr Blair of wanting to force western democracy on to Arab countries.
'When you want to go and do these regime changes, you're back to what Bush called a crusade.
'And I said, actually, in a manner of form, put on a white sheet and a red cross, and we're back to the crusades.
'It's all about religion; in these countries it's gone on for a thousand years.'
The former Labour minister Clare Short said Mr Blair had been 'absolutely, consistently wrong, wrong, wrong' on the issue, and branded him a 'complete American neocon'.
'More bombing will not solve it, it will just exacerbate it,' she said.
Ukip leader Nigel Farage dismissed Mr Blair as an 'embarrassment' who should hold his tongue.
'The lesson is not, as Mr Blair implies, that the West should intervene in Syria, let alone once more in Iraq.
'The lesson is that the West should declare an end to the era of military intervention abroad.'
Critics of the 2003 invasion have claimed that Saddam Hussein held the country's ethnic and religious groups together and provided security, but Mr Blair this morning insisted Iraq would now be in the same state as Syria if he had not intervened.
Scotland's First Minister Alex Salmond added his voice to the chorus accusing Mr Blair of 'breathtaking amnesia' over his reasons for invading Iraq.
He said: 'Tony Blair has now claimed that the invasion of Iraq was about whether or not Saddam Hussein remained in power. Eleven years ago he said it was about weapons of mass destruction.
'No reinterpretation of history will absolve the former prime minister of a direct line of responsibility for this sequence of disasters.'
And Sir Christopher Meyer, Britain's ambassador to the US from 1997 to 2003, said the decision to topple Saddam was 'perhaps the most significant reason' for the sectarian violence now ripping through Iraq.
'There are many reasons for this disastrous state of affairs. Perhaps the most significant is the decision taken more than 10 years ago by President George W Bush and Prime Minister Tony Blair to unseat Saddam Hussein without thinking through the consequences for Iraq of the dictator's removal.'
The decision to bar members of Saddam's Ba'ath party from top jobs and disbanding the army - one former member of which now leads the Isis surge, were among the most serious errors, he said.
'We are reaping what we sowed in 2003. This is not hindsight. We knew in the run-up to war that the overthrow of Saddam Hussein would seriously destabilise Iraq after 24 years of his iron rule.
'For all his evil, he kept a lid on sectarian violence. Bush and Blair were repeatedly warned by their advisers and diplomats to make dispositions accordingly.
'But, as we now know, very little was done until the last minute; and what was done... simply made things far worse.'
Last night Mr Blair published an essay online, defending his decision to go to war - and arguing that the current violence is the 'predictable' result of the West's failure to intervene in Syria.
US president Barack Obama is considering a range of military options - said to include air strikes - over the coming days.
Meanwhile Iran - a key ally of Nouri al-Maliki's Shia-dominated administration in Baghdad - indicated that it was ready to provide assistance.
Iranian president Hassan Rouhani said his country had 'no option but to confront terrorism' and was 'ready to provide assistance within international law' if requested.
And Britain is to give £3 million of aid to Iraq. International Development Secretary Justine Greening said the initial tranche of emergency cash would allow agencies to supply water, sanitation, medicine, hygiene kits and basic household items.
Mr Blair rejected as 'bizarre' arguments that Iraq would be more stable and peaceful today if the US-backed war, which claimed the lives of 179 UK personnel, had not happened.
He said that unless the international community was prepared to overcome public reluctance and confront the extremists 'hard, with force', the consequences would be more serious still.
Mr Blair - now a Middle East peace envoy - said Iraq was 'in mortal danger' but pinned the blame on the sectarianism of the Maliki government and the spread of Syria's brutal three-year civil war.
He hit back at critics of the controversial war in an eight-page essay on his website, saying al-Qaida had been a 'beaten force' in Iraq as recently as three years ago, but the chance of peace was squandered by Baghdad.
By all means argue about the wisdom of earlier decisions,' he wrote.
'But it is the decisions now that will matter.
'The choices are all pretty ugly, it is true. But for three years we have watched Syria descend into the abyss and as it is going down, it is slowly but surely wrapping its cords around us, pulling us down with it.
'We have to put aside the differences of the past and act now to save the future.
'Where the extremists are fighting, they have to be countered hard, with force.'
It did not mean another invasion, he said.
'There are masses of responses we can make short of that. But they need to know that wherever they're engaged in terror, we will be hitting them.'
He said the Iraq assault had been planned and prepared in the Syria conflict zone and was the 'predictable and malign effect' of allowing that situation to be exploited by extremists.
Air strikes against the Assad regime had been on the table last summer over its use of chemical weapons but a Commons vote against the idea helped end the prospect of action.
'I understand all the reasons following Afghanistan and Iraq why public opinion was so hostile to involvement,' Mr Blair wrote.
'Action in Syria did not and need not be as in those military engagements. But every time we put off action, the action we will be forced to take will be ultimately greater.'
He wrote: 'The moderate and sensible elements of the Syria opposition should be given the support they need; Assad should know he cannot win an outright victory; and the extremist groups, whether in Syria or Iraq, should be targeted, in coordination and with the agreement of the Arab countries.
'However unpalatable this may seem, the alternative is worse.'
- Daily Mail