Taking questions following a speech at Mansion House organised by the World Ecomomic Forum, Mr Cameron said he suspected Abu Bakr al-Baghdadi, the terror group’s leader, would be happy with Brexit.

Mr Cameron has said in the past that leaving the EU could play into the hands of Britain’s enemies, but this is thought to be the first time he has suggested Isis would actively want to see the UK leave the EU.

The Prime Minister was responding to criticisms of a speech he gave last week, in which he said a new conflict in Europe could not be ruled out. He urged voters not to take a risk on Brexit, saying: “I never said if we leave on Thursday, World War Three breaks out on Friday”.

However, he added that Russian President Vladimir Putin would welcome Brexit, adding “I suspect al-Baghdadi” would, too.

The claim will inevitably lead to further allegations of scaremongering by the Government. However, both campaigns have highlighted the ramifications of leaving or remaining in the EU for Britain’s security against terror threats.

What has the EU ever done for us? Show all 7 1 /7 What has the EU ever done for us? What has the EU ever done for us? 1. It gives you freedom to live, work and retire anywhere in Europe As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states. What has the EU ever done for us? 2. It sustains millions of jobs A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU. What has the EU ever done for us? 3. Your holiday is much easier - and safer Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed. What has the EU ever done for us? 4. It means you're less likely to get ripped off Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe. What has the EU ever done for us? 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime. What has the EU ever done for us? 6. Our businesses depend on it According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business. What has the EU ever done for us? 7. We have greater influence Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.

The Leave camp says that Brexit would give the UK greater control over its borders.

The Remain campaign contests this, saying we already have the power to check all passports, and argue that pan-European security networks including Europol and the European Arrest Warrant help keep us safe, and would be put at risk by Brexit.

Earlier, Boris Johnson was accused of "crossing the boundaries" after he made a dangerous comparison between Brussels and Adolf Hitler, the European Council President has said.

EU vote campaigns continue

Donald Tusk, the former Polish Prime Minister, attacked the former London mayor for "political amnesia" over his controversial suggestion likening the EU to the Nazi dictator's plans for domination of the continent.

It comes after the Chancellor George Osborne endorsed the view of the former military chief Field Marshal Lord Bramall, who described Mr Johnson’s remarks as "simply laughable" and "absurd".

The former Mayor had told the Sunday Telegraph that the EU was an attempt to recreate the Roman Empire’s united Europe. “Napoleon, Hitler, various people tried this out, and it ends tragically,” he said.

In the extraordinary intervention, Mr Tusk said he could not "remain silent" in the face of comments by such an "influential" politician.

Mr Tusk added: "When I hear the EU being compared to the plans and projects of Adolf Hitler I cannot remain silent. Such absurd arguments should be completely ignored if they hadn't been formulated by one of the most influential politicians in the ruling party.

"Boris Johnson crossed the boundaries of a rational discourse, demonstrating political amnesia. In some sense, he illustrated a state of mind and emotions of many Europeans, not only from the UK."

But Mr Johnson defended his comments, saying: “Over the last few thousand years there have been all sorts of attempts in Europe to recreate the dream of the Roman Empire and very often that’s been done by force. The EU is different – it’s tried to do it in a more bureaucratic way.

The 10 happiest countries in Europe Show all 10 1 /10 The 10 happiest countries in Europe The 10 happiest countries in Europe Denmark Coulourful houses and boats seen in the Nyhavn district in Copenhagen The 10 happiest countries in Europe Switzerland The 10 happiest countries in Europe Iceland Iceland, Northern Lights The 10 happiest countries in Europe Norway Wheel deal: cycling in Norway Visit Norway The 10 happiest countries in Europe Finland A girls holds a Finnish flag on the first day of the 2012 European Athletics Championships at the Olympic Stadium in Helsinki on June 27, 2012. Getty The 10 happiest countries in Europe The Netherlands The 10 happiest countries in Europe Sweden Seagulls rests on the partly frozen water near Riddarholmen in Stockholm, on February 3, 2012. Temperatures plunged to new lows in Europe where a week-long cold snap has now claimed more than 220 lives and forecasters warned that the big freeze would tighten its grip at the weekend. AFP PHOTO/JONATHAN NACKSTRAND (Photo credit should read JONATHAN NACKSTRAND/AFP/Getty Images) AFP The 10 happiest countries in Europe Austria HERRENHAEUSER, AUSTRIA - AUGUST 08: (EDITOR'S NOTE: Photo taken with an iPhone 6.) A hiker, the wife of the photographer, on day two of a four-day 50km hike across the Karwendel mountain range, walks towards Falkenhuette hut on August 8, 2015 near Herrenhaeuser, Austria. The Karwendel mountain range, part of the Austrian Alps, is located in central Tyrol and is a popular summer destination for mountain bikers, climbers and hikers. Mountain huts operated by alpine clubs and scattered across the region offer food and shelter. (Photo by Sean Gallup/Getty Images) Sean Gallup/Getty Images The 10 happiest countries in Europe Germany Getty Images The 10 happiest countries in Europe Belgium The city hall on Brussels' Grand Place is illuminated during a light show, December 30, 2015 Reuters

“The problem is there isn’t a single charismatic authority that anyone feels any loyalty to and it’s completely antidemocratic, that’s the problem.

“This discussion is bedevilled by all sorts of artificial media twit-storms or hysteria of one kind or another. There’s a very good argument against the lack of democracy in the EU.