LONDON (Reuters) - The European Union is not a "natural contributor to national security" and a British exit from the bloc would have a limited impact on U.S. intelligence cooperation, the former head of the U.S. Central Intelligence Agency (CIA) has said.
The issue of security has become one of the main arguments between those who want Britons to vote to stay in the EU and those who want to leave ahead of a June 23 referendum on British membership.
Attacks in Brussels on Tuesday have fueled the differences, with advocates for leaving arguing the bloc's open border policy allowed for the killings to take place while Prime Minister David Cameron and others said recent attacks showed greater intelligence cooperation was needed.
"The union is not a natural contributor to national security of each of the entity states and in fact in some ways gets in the way of the state's providing security for its own citizens," Michael Hayden, ex-director of the CIA and the National Security Agency, told BBC radio in an interview aired on Saturday.
Asked if a British EU exit, or "Brexit", would upset U.S. intelligence officials, he said: "With regard to this specific line of national security ... it wouldn't have much effect certainly on the American ability to cooperate with national security services."
Hayden said he agreed with comments published on Thursday from Richard Dearlove, the former head of Britain's MI6 foreign intelligence service, who said leaving the EU would do little damage to British national security and could reap security gains.
"Whether one is an enthusiastic European or not, the truth about Brexit from a national security perspective is that the cost to Britain would be low," Dearlove wrote in Prospect magazine.
However, the head of EU-wide police body Europol and other former British security chiefs have argued intelligence sharing would suffer and it would be harder for British police to protect citizens from militant attacks if it left the bloc.
Hayden praised British, French and Scandinavian intelligence agencies, but said most other European security services were small and Belgium's was "under-resourced, legally limited and frankly working for a government that itself has its own challenges in terms of overall governance".
Belgian security services have been criticized for lapses ahead of this week's attacks in Brussels which killed at least 31 people, prompting its interior and justice ministers to offer to resign.
(Reporting by Michael Holden; Editing by Mark Potter)