A former top US general has warned a potential Brexit would weaken the West in the war on terror.
General David Petraeus said that Britain leaving the EU would "deal a significant blow to [it’s] strength and resilience at exactly the moment when the West is under attack from multiple directions".
He said that none of the UK’s security problems would become easier to solve if they left the union and said he feared "that a 'Brexit' would only make our world even more dangerous and difficult to manage".
The former CIA director said: "In a time of such challenges, when our adversaries seek to sow division in our ranks and undermine the world order that we have sacrificed so much to build and defend, it is critical for the West to stand strong and united.
"And that, in turn, requires strong and principled American and British leadership inside the West.
"A Brexit would also reduce considerably Great Britain's ability to influence and guide the future of Europe, still the world's largest economic bloc; it would undoubtedly reduce British influence on the world stage, as well".
Writing in the Sunday Telegraph, he urged voters to "think twice before withdrawing from one of the most important institutions that undergirds Western strength: the European Union".
He understood the appeal of isolationism in both the UK and the US but said history shows it is "a strategic dead end".
What has the EU ever done for us?
What has the EU ever done for us?
1/7 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.
2/7 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.
3/7 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.
4/7 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.
5/7 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.
6/7 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.
7/7 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 retired four-star army general, who commanded forces in Iraq and Afghanistan, said "Some have suggested that leaving the EU would reduce the risk of terrorism in the UK. That is mistaken. The terrorist threat, unfortunately, will be with all of us for the foreseeable future, as the recent events in Belgium have just shown.
"In fact, the best way to defend ourselves is precisely by deepening military, intelligence, and diplomatic co-operation across the Western world, by working together with our partners on the continent and elsewhere to strike at the terrorists in their sanctuaries and tackle the underlying drivers of radicalisation.
"The EU, for all its imperfections, is a vital player in this effort."