Twelve former senior military officers insisted Britain should stay in the European Union Wednesday, saying the country was "stronger" as a member of the 28-nation bloc in a "dangerous world".

Writing ahead of a membership referendum on June 23, the retired military chiefs said the EU helped protect the country against threats from Islamic State (IS) jihadists and Russia.

The signatories of the letter to the Daily Telegraph newspaper include four former professional heads of the overall armed forces as well as two former heads of the army and two of the navy.

"Will Britain be safer inside the EU or outside it? When we look at the world today, there seems to us only one answer," the letter said. "Within the EU, we are stronger."

The letter added that, while NATO was the most important alliance for national security, the EU was "increasingly important".

"Europe is facing a series of grave security challenges, from instability in the Middle East and the rise of Islamic State of Iraq and the Levant, to resurgent Russian nationalism and aggression," it added.

"In a dangerous world, it helps us to safeguard our people, our prosperity and our way of life. We therefore believe strongly that it is in our national interest to remain an EU member."

The number of military chiefs who signed the letter was initially put at 13 before Prime Minister David Cameron's Downing Street office, which organised it, clarified that one of those whose name was put to the letter had not actually signed.

Cameron secured a deal Friday to renegotiate Britain's relationship with the EU ahead of the referendum, triggering the start of campaigning.

But his efforts to keep Britain in the EU suffered a blow on Sunday when popular London Mayor Boris Johnson came out in favour of leaving.

A YouGov opinion poll for the Times newspaper Wednesday indicated that 38 percent of voters surveyed wanted to leave the EU while 37 percent wanted to remain and 25 percent were undecided.