David Petraeus (pictured) warned British voters today that leaving the EU would do nothing to protect them from future terrorist attacks

Ex-CIA director David Petraeus warned British voters today that leaving the EU would do nothing to protect them from future terrorist attacks.

In the wake of the Brussels attack last week – the second deadly terrorist atrocity to hit a European capital within five months – the retired four-star US general said it would be the worst time for Britain and the EU if voters opted to leave in June’s referendum.

General Petraeus, who led US and Nato forces in Iraq and Afghanistan, said it would ‘deal a significant blow to the EU’s strength and resilience at exactly the moment when the West is under attack from multiple directions’.

His comments directly contradict his fellow ex-CIA director General Michael Hayden, who said on Friday that Britain’s national security will not be harmed if it leaves the EU.

He said membership of the 28-state bloc ‘in some ways gets in the way of the state providing security for its own citizens’ and dismissed claims by In campaigners that the UK would lose out from shared intelligence if it voted to leave.

And he warned that the UK should not rely on the poor intelligence services of many EU member states such as Belgium, whose authorities failed to stop home-grown terrorists kill 31 people last week.

National security matters have taken an increasingly important role in the EU debate this week following the deadly attacks in Brussels and spy chiefs are split over whether leaving the 28-state bloc would damage or benefit Britain’s national security.

General Hayden is among a number of former spy chiefs, including former Mi6 chief Sir Richard Dearlove, who have backed Britain leaving the EU, insisting it would not damage national security and would bring benefits for Britain’s security agencies.

General Petraeus today joined others such as former GCHQ chief Sir David Omand in dismissing the argument and urged voters to ‘think twice’ before taking a potentially dangerous path.

‘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," he wrote in the Sunday Telegraph - pointing to Russian expansionism, the Middle East and cyber crime.

‘And that, in turn, requires strong and principled American and British leadership inside the West.

The comments, by ex-CIA director David Petraeus come in the wake of the Brussels attack last week (pictured)

‘For that reason, as an American with enormous respect for and appreciation of the UK and its citizens, I encourage my British friends to think twice before withdrawing from one of the most important institutions that undergirds Western strength: the European Union.

‘As an American who was privileged to command America's and the UK's sons and daughters in combat and as one very concerned about international security issues, there is no question in my mind that a 'Brexit' would deal a significant blow to the EU's strength and resilience at exactly the moment when the West is under attack from multiple directions.

‘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.

He said he understood the draw of isolationism in both the UK and US but warned: ‘History teaches us that such an approach is a strategic dead end.

‘And recalling the past reminds us that every time we have gone in an isolationist direction, the result has been calamitous for the entire world.’

He concluded: ‘There is simply no retreat from this reality. The connections are too considerable - and too consequential.

Another ex-CIA director, General Michael Hayden (pictured), said on Friday that Britain’s national security will not be harmed if it leaves the EU

‘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 Ex-CIA director David Petraeus

‘The EU, for all its imperfections, is a vital player in this effort.’

But General Hayden, speaking to the BBC on Friday, said national security should always be the responsibility of individual states and said being in the EU does not contribute much to Britain’s spying capabilities.

And in a damning assessment of the EU as a whole, he said security across the continent was ‘very uneven,’ suggesting the lack of consistency contributed to the Brussels terror attacks.

General Hayden, who was in charge of the top spying agency in the US from 2006 – 2009 and previously served as director of the National Security Agency, criticised Belgium’s security in the wake of the Brussels attacks, saying it was ‘small, under-resourced [and] legally limited’.

His comments echo former Mi6 chief Sir Richard Dearlove who on Thursday backed Britain leaving the EU, insisting it would not damage national security.

People gather around floral tributes at a memorial site at the Place de la Bourse in Brussels days after the terror attack

Sir Richard said getting out of the European Convention on Human Rights and the EU's free movement could boost security.

The pair have contradicted comments by ex-GCHQ director David Omand, who declared that Britain would be the ‘loser in security terms’ if it left the EU because it would ‘jeopardise the flows of intelligence between European nations’

Speaking to the Today programme, General Hayden said: ‘National security remains a national responsibility and so sadly the grades you have to give to each of the services are frankly individual and it’s very uneven.

‘Very good services, aggressive services in France and Great Britain, good but smaller services to in Scandinavia and then unfortunately across most of the rest of the continent – smaller services and if you’re talking specifically about Belgium – small, under-resourced, legally limited and frankly working for a government that frankly has itself has its own challenges in terms of overall governance.’

Earlier he told Sky News: ‘I don't know if the European Union contributes a great deal to ... espionage.

‘Europe has divided its governance between things done in Brussels, at the Union level, and things done back in national capitals.

‘At the Union level, they talk about commerce and privacy. But to keep citizens safe, that remains a responsibility back in national capitals.

‘And right now, because of some of the positions the Euro institutions have taken on surveillance and privacy, the capitals are finding it more difficult to provide for their own citizens' safety.

The Brussels attack was the second deadly terrorist atrocity to hit a European capital within five months. French special forces are pictured evacuating people during the Paris attacks in November

‘So to a degree, Brussels, as a Euro institution, keeps pushing these activities at the expense of security - to that degree I can understand why Sir Richard is saying what he's saying.’

In an article for Prospect Magazine, Sir Richard said national security was served by international cooperation but only with trusted allies and not the entire EU as this leaked information like a 'colander'.

He said: 'Though the UK participates in various European and Brussels-based security bodies, they are of little consequence.

'The Club de Berne, made up of European Security Services; the Club de Madrid, made up of European Intelligence Services; Europol; and the Situation Centre in the European Commission are generally speaking little more than forums for the exchange of analysis and views.

'With the exception of Europol, these bodies have no operational capacity and with 28 members of vastly varying levels of professionalism in intelligence and security, the convoy must accommodate the slowest and leakiest of the ships of state.'

Sir Richard warned: 'The larger powers cannot put their best intelligence material into such colanders.'

Sir Richard said counter terrorism work was usually conducted bilaterally or occasionally trilaterally.

His comments goes against the position of Home Secretary Theresa May (pictured), who surprised some in Westminster when she backed David Cameron's EU In campaign on the grounds of security

He said: 'Brussels has little or nothing to do with them, in large part due to what is known as the ''Third Party Rule,'' a notion that is little understood outside the intelligence fraternity but which is essential to intelligence liaison worldwide.

'This rule states that the recipient of intelligence from one nation cannot pass it on to a third without the originator's agreement.'

Sir Richard insisted this meant security cooperation with the United States would not be damaged by a Brexit. 1

And he concluded: 'In short, Europe would be the potential losers in national security.

'But if Brexit happened, the UK would almost certainly show the magnanimity not to make its European partners pay the cost.'

His comments goes against the position of Home Secretary Theresa May, who surprised some in Westminster when she backed David Cameron's EU In campaign on the grounds of security.

Earlier this week she repeated her warnings against Brexit.

In an interview with The Times, conducted before Tuesday’s Brussels attacks, Mrs May said EU membership was 'of benefit' to Britain's security.

She said: 'I think this is an issue people should look at more broadly, but on the security front there are good reasons for us to be members of the European Union.

'The UK threat level is at severe, which means a terrorist attack is highly likely. We know that since November 2014 seven terrorist plots have been disrupted in the UK.

'What's important is that we work with others to ensure that we can respond.'