Britain and its allies fuelled the rise of ISIS by failing to take action against Syrian President Bashar al-Assad in 2011 and 2013, George Osborne (pictured earlier this month in Manchester) has said

Britain and its allies fuelled the rise of ISIS by failing to take action against Syrian President Bashar al-Assad in 2011 and 2013, George Osborne has said.

The former Chancellor also admitted the Government's 'cost of not intervening' had caused the refugee crisis that had 'fuelled the rise of extremism across Europe' and allowed Russia to return as a major player in the Middle East.

He said the Government's 'conscious' decision not to intervene in 2011 was a mistake and allies could have altered the outcome of the civil war by 'forcefully backing the more moderate elements of the opposition'.

But after Assad 'crossed the red line' by allegedly using chemical weapons against civilians the Government pressed ahead with plans to intervene in Syria, only for MPs to narrowly defeat the Government in a crucial Commons vote.

Mr Osborne described it as the 'single most depressing moment of my time to date in Parliament'.

It played a crucial part in Barack Obama's decision to ditch US plans for bombing Assad forces and the West's failed attempt to find a diplomatic solution to the civil war, which has ravaged Syria for more than five years.

Mr Osborne said the alternative Britain and the US settled on was 'much weaker' than military intervention.

Failure to intervene had seen 'hundreds of thousands killed, millions displaced, neighbouring countries destablised,' Mr Osborne said in a candid lecture at the Chicago Council on Global Affairs.

But worst of all it had led to the 'emergence of a terrorist state' run by ISIS, he said.

George Osborne admitted the Government's 'conscious' decision not to take action against Syria President Bashar al-Assad (pictured giving an interview to the Syrian Arab News Agency yesterday) in 2011 had helped create a 'terrorist state' run by ISIS

George Osborne admitted that the West's failure to take action in Syria had created a refugee crisis that had destabalised neighbouring countries. Pictured, five-year old Syrian boy Omran Daqneesh sits alone in the back of an ambulance after being injured during an air strike in Aleppo on August 17 this year, believed to have been launched by Russian or Assad forces

Today the ceasefire was abandoned by the Syrian military, which launched a fresh offensive against rebels in eastern Aleppo - home to a quarter of a million civilians. Pictured, ambulances and buildings are left destroyed in Ansari, eastern Aleppo this morning after Syrian forces launched fresh airstrikes against rebels

Sharing his regrets of his time in office, Mr Osborne, who was at the centre of the Government for more than six years, said: 'We made a conscious decision not to intervene in 2011, when Britain, America and our allies could have tried to alter the outcome of the emerging civil war there by forcefully backing the more moderate elements of the opposition.

'There was a plan put forward to do that. But collectively the West chose not to take it up - and we settled on something much weaker.

'And we chose not to intervene again in 2013, when Assad crossed the red line we had drawn and used chemical weapons.

'The vote of the House of Commons against military action was the single most depressing moment of my time to date in Parliament.'

Mr Osborne was speaking as diplomatic relations over the Syria crisis hit rock bottom after US Secretary of State John Kerry and Sergei Lavrov failed to salvage a deal to rescue an agreed ceasefire over Syrian battle zones.

The Syrian civil war has raged for more than five years and diplomatic efforts to find a solution hit rock bottom after the Syrian military abandoned a ceasefire and launched fresh air strikes against rebels in Ansari, eastern Aleppo, where rescue workers were seen removing a destroyed ambulance this morning

Members of the rebel Syrian Civil Defense group known as the White Helmets assess the damage after the Syrian military launched fresh air strikes today

George Osborne said the Government's defeat in the 2013 vote on military intervention in Syria was the 'single most depressing moment of my time to date in Parliament'. Pictured, David Cameron admits defeat in the crucial Commons vote in August 2013

Earlier this week a US aid convoy was hit by an air strike - believed to have been a Russian bomb - during a one-week truce, triggering diplomatic meltdown.

And today the ceasefire was abandoned by the Syrian military, which launched a fresh offensive against rebels in eastern Aleppo - home to a quarter of a million civilians.

In his lecture - delivered on Wednesday evening, said it was impossible to know what would have happened if the West had intervened as soon as the civil war broke out in 2011 or in 2013, but said the inaction had made Western countries less secure.

'I don't know whether these interventions in Syria would have worked,' he said. 'I am sure they would have been very messy and difficult. Clinical interventions and text book nation building exist only in newspaper columns.

'But I do know what has happened in Syria while we chose not to intervene decisively. Hundreds of thousands killed. Millions displaced.

'Neighbouring countries destabilised. The taboo on the use of chemical weapons broken. The emergence of a terrorist state. Russia back as a major player in the Middle East. And a refugee crisis that has fuelled the rise of extremism across Europe.'

The West's failure to take action in Syria in 2011 and 2013 had allowed Russian president Vladimir Putin (pictured with US President Barack Obama at the G8 summit in Northern Ireland in 2013) take a stranglehold in the Middle East

In his Chicago lecture, Mr Osborne also shared his thoughts on Brexit, admitting freedom of movement will not survive Brexit as he warned that talks with the EU will not start until next autumn.

The former Chancellor, one of the biggest supporters of the EU's open borders immigration rules, said he could not envisage the Government accepting a deal that continues free movement of people because the issue 'clearly caused such concern in the referendum'.

He urged Theresa May to seek the 'closest possible' relationship with the EU on economic and security issues.