Anders Fogh Rasmussen, a former NATO General and former Dutch Prime Minister, called on the UK and the EU to push negotiations on security and defence ahead of Brexit trade talks.
Speaking to Sky News, Mr Rasmussen said the UK and the EU should start working on a “security treaty immediately”.
He said: “Brexit will not have any significant importance regarding the UK membership of NATO but it is a matter of concern that we still need real negotiations on the future of the security relationship between the UK and the European Union.
“Let’s remind ourselves that when it comes to trade you’ll always have the WTO tariffs as a fallback, but when it comes to security you don’t have any fallbacks so I would caution against waiting to negotiate the security relationship between the EU and the UK until the very end of this process.
“They should start already now.
“I appreciate that it’s not used as a bargaining chip but it is essential that we can maintain the current relationship between the UK and Europe when it comes to security.
“Whether it’s counterterrorism, free data flow, data protection, etcetera, it’s essential that we maintain the current close relationship between the UK and Europe.
“Actually, I think what we should aim at is to reach an agreement on a fully-fletched security treaty between the UK and the EU and that would take time so these negotiations should start immediately.”
Former NATO commander James Stavridis said Brexit will undermine European defence internationally as the council will lose the expert input of the UK.
Speaking to BBC Radio 4, Mr Stavridis said: “One great advantage of the United Kingdom being in the European Union from a security perspective as viewed from the United States was that your voice was in the centre of those conversations.
“And it was always a voice of stability, sensibility, forthrightness, resoluteness, the things that we associate with your nation.
“When, as an American, I see that voice come out of the defensive councils of Europe, I think Europe is the one who will be diminished, not the United Kingdom.
“I am very concerned that the Europeans will end up throwing money at the problem and simply buying forces for the sake of purchasing more defence.”