A letter signed by former military chiefs has warned leaving the EU could hamper the UK's ability to tackle military threats.
It is the second pro-EU letter part co-ordinated by Downing Street to be published in a national newspaper in two days.
On Monday almost 200 business leaders signed a letter partly-organised by Number 10 saying the UK's economy could be put at risk by Brexit.
In the letter to the Telegraph newspaper, 13 former armed forces bosses from across the Army, Navy and RAF warn Europe faces "grave threats" from the likes of Islamic State and Russia.
It concludes that Britain is in a "stronger" position to deal with the threats by staying in the EU.
We are proud to have served our country and to have played our part in keeping Britain safe. In the forthcoming referendum, therefore, we are particularly concerned with one central question: will Britain be safer inside the EU or outside it? When we look at the world today, there seems to us only one answer. Europe today is facing a series of grave security challenges, from instability in the Middle East and the rise of Daesh, to resurgent Russian nationalism and aggression. Britain will have to confront these challenges whether it is inside or outside the EU. But within the EU, we are stronger. Inside it, we can continue to collaborate closely with our European allies, just as we did when we helped to force the Iranians to the negotiating table through EU-wide sanctions, or made sure that Putin would pay a price for his aggression in Ukraine.
Former defence chiefs
Signatories include include former chiefs of defence staff Field Marshal Lord Bramall, Field Marshal Lord Guthrie and General Sir Mike Jackson, the former Chief of the General Staff.
Air Chief Marshal Lord Stirrup, Admiral of the Fleet Lord Boyce and former special forces chief General Sir Michael Rose are also among the 13 senior officers who backed the letter.