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.
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.