Former head of MI6, Sir Richard Dearlove, has said leaving the European Union could boost UK security.
His comments directly contradict those made by Home Secretary Theresa May who has said being a member of the EU helped "enhance" security.
Writing in Prospect magazine, Sir Richard also suggested that there were "vastly varying levels of professionalism in intelligence and security" across the EU's 28 member states and suggested that some of them leaked like "colanders".
The Former Secret Intelligence Service chief added: "Would Brexit damage our defence and intelligence relationship with the United States, which outweighs anything European by many factors of 10? I conclude confidently that no, it would not."
The European Convention on Human Rights is not an EU creation, instead it covers the Council of Europe members.
Sir Richard said "few would notice" the loss of the European Arrest Warrant, and insisted that the UK's security and intelligence relationship with the rest of Europe would probably hardly change at all.
However, Defence Secretary Michael Fallon told Channel 4 News he did not accept Sir Richard's analysis.
He said: "What is very clear to me as the Defence Secretary charged with helping to keep this country safe ... is that we should not be leaving international organisations or intelligence-sharing partnerships, that would be exactly the wrong thing to do.
"On the contrary, when you are faced with a terrorist threat we should be sharing as much information as possible and we should be coming to the aid of our allies and friends rather than walking out on them."