Leaving the European Union could make Britain more vulnerable to terrorism attacks, according to two former senior British intelligence chiefs.
Sir John Sawers, who stepped down as head of MI6 in 2014, and Jonathan Evans, head of MI5 until 2013, warned that Brexit could weaken intelligence-sharing between Britain and neighbouring countries.
"Counterterrorism is a team game, and the EU is the best framework available - no country can succeed on its own," they wrote in the article.
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 union bloc.
They added that 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.
Speaking on the BBC's Andrew Marr show Sawers also added that as a country "we are only secure because the wider Europe is secure" and that staying in the EU was "about the wider stability of our continent".