More than a dozen senior British military leaders are to sign a letter that will warn voters not to support leaving the EU in an upcoming referendum because doing so would compromise the UK’s national security, the Daily Telegraph reported.

The letter, to be published this week, is being organized by Downing Street and will cite the threat from the terrorist group Islamic State (IS, formerly ISIS/ISIL) and “increasing Russian aggression” as the reasons why Britain should remain part of the European Union, the British newspaper said. Those likely to sign the letter include Admiral Lord Boyce, General Lord Stirrup and Field Marshal Lord Bramall, three former Chiefs of the Defence Staff, and General Sir Peter Wall, a former Chief of General Staff. Others are reluctant to endorse the message, the Telegraph said.

“Yes there is a security dimension to the EU, but in my mind it is more of a policing and judicial matter rather than a military matter. The military dimension is provided by NATO,” one of them, General Sir Mike Jackson, a former head of the British Army, told the newspaper.

The UK is to hold a referendum on “Brexit” on June 23, PM David Cameron announced last week, after securing a deal ensuring special treatment for the UK within the union. His cabinet largely backs remaining part of the EU and national security is one of the arguments it uses to support the position.

Defence Secretary Michael Fallon said in an article for the Sunday Telegraph that Russian President Vladimir Putin wants Britain to leave the European Union. “It is not scaremongering to ask which result Putin would favor. If we left, the European Union for the first time in its history would be smaller and weaker. That’s obviously in Russia’s interests,” he argued.

Some members of the cabinet, however, are challenging the security argument. Justice Secretary Michael Gove said in a statement that EU politicians are allowing terrorists to roam freely in their countries, which fuels the rise of far right-wing parties and opens up historic tensions between members of the union. London Mayor Boris Johnson is another Conservative heavyweight openly favoring an out vote in the referendum.