Britain's national security will be severely weakened if the UK leaves the EU without a deal and it could take "years and years" to rebuild.

The blunt warning comes from three men who have served at the very top of the British defence and security establishment: Sir John Sawers, the former head of MI6, Lord David Richards, the former Chief of the Defence Staff, and Lord Peter Ricketts, the former National Security Adviser.

Speaking exclusively to Sky News, they all said that Britain would lose access to aspects of the European security framework, whatever form Brexit eventually takes.

"Any form of Brexit makes our security more difficult to manage... the harder the Brexit, the greater the damage," said Sir John Sawers in a rare television interview."We've been the only country in the world that has been a member of NATO, a member of the European Union and a member of the Five Eyes intelligence alliance.

Image: The warnings come from former heads of security and defence

"It's given Britain a uniquely powerful and influential position in the world. We're now kicking away one of those pillars by leaving the European Union."


As well as serving as 'C' between 2009-2014, Sir John was also the British ambassador to the United Nations. He believes a second referendum might be necessary.

"The government is operating under a mandate from the referendum. It needs to negotiate the best possible deal it can and get that through parliament. But if that fails and we're left to leave the European Union without a deal, that would be really, really damaging for this country and I think then there would be a very strong case to putting that back to a second referendum."

Image: Lord Ricketts says security relationships could take years to rebuild

Lord Ricketts was the National Security Adviser under David Cameron and also served as the British ambassador to France from 2012 to 2016.

He said if Britain leaves the EU without a deal, security relationships could take "years and years" to rebuild.

He warned that a "no-deal" Brexit would create "a really serious and immediate problem for British national security".

"My own view is that the best outcome for British national security is to stay in the EU, that gives us full access to all the range of instruments.

"The next best is to have a negotiated arrangement as a third country, which won't be as good but is a lot better than nothing. The worst of all is to crash out with no-deal."

But another former head of MI6, Sir Richard Dearlove, recently wrote to Conservative party chairman warning against backing the prime minister's deal.

"The first duty of the state, above trade, is the security of its citizens. The Withdrawal Agreement abrogates this fundamental contract and would place control of aspects of our national security in foreign hands." he said.

Currently as a member of the EU, Britain is also a member of Europol, the European Arrest Warrant and the Schengen Information System known as SIS 2.

The latter allows passport data to be transferred between countries, helping security forces track criminals or persons of interest.

Once outside the EU, the UK will automatically leave these bodies and will have to renegotiate access, probably on a reduced scale.

Image: Lord Richards has urged MPs to back Theresa May's deal

Lord Richards led the British military from 2010 to 2013. He has urged MPs to back Theresa May's deal for the sake of national interests.

"Crashing out is not a good option. A deal is best, going out without one will lead to short-term disruption. We must not, in this case, allow excellence to be the enemy of good enough which gives us time to build our position for future negotiations."

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MI5, MI6 and GCHQ have helped disrupt terrorist attacks in Europe in recent years, and in no small part, that's been down to the UK's close relationship with the United States.

In Lord Richards' view the EU needs UK security co-operation every bit as much as the other way around.

"Our access in particular to US intelligence feeds, which we can feed discreetly where appropriate and with American compliance or with American support into Europe to bolster their counter-jihadist campaigns, is something the European nations in particular, prize."