A British vote to leave the European Union next month could make the country more vulnerable to militant attacks and cause instability across the continent, two former senior British intelligence officials said.

John Sawers, who stepped down as head of the MI6 foreign intelligence service in 2014, and Jonathan Evans, who led the MI5 domestic spy agency until three years ago, warned that a British exit, or Brexit, could weaken intelligence-sharing between Britain and neighboring countries.

"Counterterrorism is a team game, and the EU is the best framework available — no country can succeed on its own," they said in an article for the Sunday Times newspaper.