LONDON (Reuters) - 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.
National security has become a key area of contention between rival campaigners ahead of the June 23 vote, particularly in the light of Islamic State attacks in Paris and Brussels.
Those wanting Britain to leave the 28-member bloc say an exit would enable the nation to have greater control over its borders. Those backing membership, including Prime Minister David Cameron, say the EU helps to coordinate intelligence-sharing.
Sawers and Evans said modern intelligence work relied on the sharing of large data-sets and that Britain could be restricted in the information it received if it was no longer part of the bloc.
The two men, who do not often speak out on national matters, said their concerns about the vote went beyond Britain's security and that the removal of one of Europe's main military powers could unsettle the EU itself.
"If the UK were to withdraw from the EU, the destabilizing effect on the EU itself — already beset with economic difficulties, the migration crisis and a resurgent Russia — could be profound," they said.
"Those who are enemies of democracy would rejoice. In our judgment, there is a real risk that such a detribalization could, in time, lead to the fragmentation of the EU and the return of instability on the continent."
(Reporting by Kate Holton; Editing by Lisa Von Ahn)