Former Labour foreign secretary Lord Owen is calling for the UK to leave the EU, saying remaining in the union is "the more dangerous option for British security".
The ex-minister told The Sun EU law over-rides UK law and there were "many positive aspects" to leaving.
His intervention comes the day after 12 former defence chiefs said the UK was safer in the EU.
The UK's referendum on its EU membership takes place on 23 June.
Lord Owen, who was foreign secretary between 1977 and 1979, was one of the so-called "gang of four" who quit Labour in 1981 to set up the Social Democratic Party.
He sits as an independent in the House of Lords having given up his crossbench status in 2014 to donate to Labour.
In his Sun article the peer, who suggested he could back an exit in September, said leaving the EU "could be the spark we need to re-energise our nation: a challenge and an opportunity".
"To remain in the EU is in my judgement a more dangerous option for British security in its deepest sense - economic, political, military and social - than remaining in a dysfunctional EU dragged down by a failing eurozone," he added.
"Remaining in the EU is risking more than leaving."
However, Defence Secretary Michael Fallon has said leaving the EU would be a "big gamble" with the UK's security.
Mr Fallon said the "collective weight" of partnerships such as the EU made it easier to deal with global threats,