Getty Russia and ISIS are said to be the two major threats

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Three former Chiefs of the Defence Staff, Admiral Lord Boyce, General Lord Stirrup and Field Marshal Lord Bramall, are all among a group of more than a dozen set to sign a letter to be sent this week saying Britain’s national security relies on the EU. An increasing number of ISIS attacks and mounting Russian aggression will be stated as two of the main threats.

Other former military chiefs are still deciding whether to support the letter as they battle with personal thoughts over national security and anti-EU feelings. Downing Street is said to be organising the letter as both Prime Minister David Cameron and Defence Secretary Michael Fallon push the national security argument to remain in the EU.

Getty Michael Fallon has joined David Cameron in pushing the national security argument

Mr Cameron suffered a blow today after close friend and Mayor of London Boris Johnson announced he will be campaigning to leave the EU two days after Mr Cameron closed a deal with other European leaders to reform Britain’s membership of the EU. Lord Stirrup told The Daily Telegraph: “I don't carry a torch for the European Union at all but one has to look at the realistic alternative not just the World as we wish it to be. “In light of the current threats like ISIS, Russia and other threats that might emerge you have to think about how we secure our society."

Lord Bramall told the newspaper: "I have always felt that a strong Europe in political terms is infinitely stronger if it has Britain inside it. "If Britain left it would be a much weaker Europe and therefore it would affect the whole balance of power and equilibrium in the Western World. "That affects not just security, but the political side. The negotiations. It is important to have a match for the various power blocs - China, Russia - it's complementary to Nato. I am sure America would very much want us to be in."

Getty Lord Stirrup is calling to remain in the EU

IG The threat of ISIS is ever-growing, the military chiefs have warned

General Sir Mike Jackson, former head of the British Army, does not yet know if he will sign the letter, saying he views the EU as moulding policing and judicial matters rather than military concerns. News of the letter comes just hours after it was revealed about 50 FTSE-100 chiefs will publish a letter warning Britain’s economy will suffer if the UK leaves the EU.