Sweden is home to at least 2,000 ISIL sympathisers who are believed to have been radicalised over the internet, the country’s spy chief revealed on Monday.

Anders Thornberg, who heads the domestic intelligence agency Säpo, said the number of ISIL loyalists had increased from a suspected 200 in 2010; a 10-fold leap.

“We have never seen anything like it before,” Mr Thornberg told the Swedish news agency TT. “We would say that it has gone from hundreds to thousands now.

“This is the 'new normal' … It is an historic challenge that extremist circles are growing," he said.

Mr Thornberg said the emergence of ISIL, with its sophisticated propaganda machine that has made effective use of the internet and social media channels, had given previously disparate groups of extremists a common cause to unite around.

“We used to have different circles. We had radicalised people from North Africa, the Middle East and Somalia, but they were all separate,” he said.

Mr Thornberg also reassured Swedes that the security service believed very few of the number had either the intention or the wherewithal to carry out an attack. But experts believe the pressures are immense and the dangers are growing.

"There has been a tsunami of carefully calibrated propaganda and recruitment – and with social media ISIL can reach out to anyone's living room,” said Magnus Ranstorp, a terrorism expert at the Swedish Defence University.

In April this year, an Uzbek national with professed sympathies for ISIL used a stolen lorry to run over shoppers in the centre of Stockholm, killing five and injuring 15. And Swedish national Osama Krayem was charged with murder over his involvement in the March 2016 Brussels terror attacks which claimed more than 30 lives.

Mr Thornberg said that Swedish police were now dealing with close to 70,000 leads a year relating to terrorism and extremism, up from 24,000 five years ago.

Mr Ranstorp told Swedish news website The Local that the new figure of 2,000 extremists was what many had expected.

"Those of us working in the field of counter-terrorism are not surprised, it has been in the making for some time, it didn't just appear. When ISIL declared the caliphate, that was the genie out of the bottle," he said.

"It is a development we're seeing in general, of course it's worse in bigger countries. In the UK there are 23,000 extremists, in Belgium there are 18,000, in France 17,000. Not everyone is equally dangerous, but it only takes a small number of people."

There are variety of reasons for the increasing numbers, Mr Ranstorp said. “First, there's the mobilisation within the context of the conflict in the Middle East, in particular Syria. Then there's social media."

The growth of religious extremism in areas of Sweden with high poverty levels and crime rates, which took place among segregated immigrant communities, was a third reason, according to Mr Ranstorp.

“And the fourth reason is maybe that the prevention measures have been pretty tame. It's become better in the last year, but I have long said that if you compare Denmark and Sweden, Denmark is at university level and Sweden at kindergarten level,” Mr Ranstorp told The Local, calling on national and local authorities to better integrate communities that were providing support for Islamist extremists.