Robertson says if the EU wants to use NATO assets, such as airborne warning and control aircraft, intelligence and communications, they will have to use the NATO military planning facility at supreme allied headquarters in Belgium.

CNN: NATO battles EU over defence

Speaking to BBC Radio 4's World at One programme from Nato's headquarters in Belgium, Sir Richard - the deputy supreme allied commander in Europe - said green-on-blue attacks were "an issue that we have to come to grips with".

BBC: General Sir Richard Shirreff