Thousands of jihadis being held in Syria pose a ‘chronic’ threat to the West’s security if they are released, the former chief of MI6 has warned.

Sir Alex Younger said yesterday the toppling of the Assad regime risked a ‘serious spike’ in the threat of ‘a very large number’ of Islamic State (IS) detainees becoming free.

There are fears a flood of extremists could head to Europe if security around their detention camps is relaxed.

It comes as Foreign Secretary David Lammy cautioned a Syrian civil war could trigger a new migration crisis for the UK and Europe.

He suggested the flow of joyful Syrian refugees returning home ‘could quickly become a flow back out’ if the revolution backfired.

Mr Lammy branded deposed tyrant Bashar al-Assad ‘the rat of Damascus, fleeing to Moscow with his tail between his legs’ as the Kremlin confirmed Vladimir Putin had personally granted him asylum on ‘humanitarian grounds’.

After his bloodthirsty 24-year rule, Assad escaped by private jet with his British-born wife Asma and their children as the rebels closed in on Damascus at the weekend.

 With an estimated $2 billion (£1.6 billion) fortune looted from the Syrian people, the Assads will live as exiles in Moscow, where they are said to own 18 luxury apartments in what was once Europe’s tallest building.

Assad’s in-laws left the family’s £1 million west London home ten days ago, neighbours said yesterday. As the Middle East turmoil continued:

– Jubilant refugees in London vowed to go home to Syria as they celebrated the overthrow of Assad

– The rebels’ three-stars flag flew at Syria’s embassies in London and Moscow

– Britain, Germany and Austria all froze Syrian asylum applications

– Syrian families headed to torture jails desperately trying to find relatives imprisoned by the regime

– Horrors uncovered included an ‘iron press’ used to crush prisoners

– Israeli tanks rumbled into disputed Golan Heights territory and its warplanes struck suspected chemical weapons depots in Syria

– Western leaders called for a peaceful political transition

Amid the chaos, with humiliated Putin vying with Turkey, the US, Iran, Israel and Islamists for control of Syria, there were fears the hated Islamic State terror group could rise again to wage a new campaign of terror.

A decade ago, IS fighters conquered swathes of the Middle Eastern nation, with their beheadings of Western hostages appalling the world. 

Their medieval-style regime was eventually defeated and thousands of jihadi extremists were imprisoned by Kurdish groups, with US help – but many might now go free following the overthrow of 59-year-old Assad.

Sir Alex, who ran MI6 from 2014 to 2020, warned of a ‘chronic threat to our integrity and security’.

He told BBC Radio 4’s Today programme it was for Syrians to decide their own future but Britain had some ‘hard interests’.

‘The primary one of those is the existence of a very large number of IS detainees left over from the destruction of the caliphate, currently contained by the Kurdish groups in the east.

 If they go ‘off the job’, you can expect a serious spike in the threat posed to Europe by IS.’

 About 900 American troops stationed in Syria help to keep the jihadis in secure camps, and Sir Alex said: ‘If I were to have five minutes with Donald Trump, I would explain that the small number of US troops in eastern Syria are playing an absolutely vital role in allowing the Kurdish groups to stay on the case, in terms of constraining IS.

‘If he does withdraw those troops, and he may well, it’s going to present Europe with a conundrum.’

Former IS members being held in Syrian prison camps include Shamima Begum – the former London schoolgirl who ran away to join the terror group – and British-born ‘Jihadi Jack’ Letts, both of whom were stripped of British citizenship.

While relative calm returned to the Syrian capital Damascus yesterday, fighting raged elsewhere as groups vied for territory and power. 

Many clashes were in Kurdish-controlled areas in northern Syria, where Turkish-backed militants have taken advantage of the disarray.

It is feared these skirmishes could give ISIS prisoners a chance to break out.

It is unclear if the new rulers in Damascus – Hayat Tahrir al-Sham (HTS) – will help Islamic State members.

HTS leader Abu Mohammed al-Jolani – a former Al Qaeda devotee – yesterday shed his nom de guerre, switching to his real name Ahmed al-Shara at the Umayyad mosque in Damascus, where he proclaimed unity by saying: ‘By God’s grace, this land of Syria was liberated within 11 days from its north to its south, from its east to its west.’

A day after ousting Assad, his Islamist group appeared to be trying to restore a sense of order to the capital.

At the weekend, Assad’s palaces were ransacked, statues torn down and some banks looted. Yesterday, groups dressed in mismatched camouflage with rifles slung over their shoulders were stationed outside government buildings and banks.

The rebels have granted amnesty to their former foes – soldiers who were conscripted into service during Assad’s rule. ‘Their lives are safe and no assault on them is permitted,’ the group said.