They said the UK would be in a “stronger” position to tackle the crazed terror group – which has threatened more attacks in Europe – by saying in the EU.
Thirteen former Royal Navy, Army and RAF officials made the bold claim in a letter backed by Downing Street.
It comes just days after David Cameron revealed a referendum on Britain”s membership of the EU will be held in June.
The chiefs went on to highlight “grave security challenges” faced by Europe including ISIS and Russian “aggression”.
They added: “Britain will have to confront these challenges whether it is inside or outside the EU. But within the EU, we are stronger.”
The letter was signed by top military officials including Field Marshal Lord Bramall and General Sir Michael Rose.