Britain will continue to block a European Union army as long as it remains in the bloc as it undermines NATO military alliance, Defence Secretary Michael Fallon said.
But speaking at the Conservative party conference in Birmingham on Tuesday, he said: "Leaving the EU does not mean we are stepping back from our commitment to the security of our continent."
France and Germany hope that Britain's vote to leave the EU will leave the path open for common defence proposals.
Making a swipe at Labour leader Jeremy Corbyn, known for his anti-nuclear arms stance, he said: "Waving a white flag, will not keep us safe.
"Labour would risk our defence for their dogma. That's why they can never be trusted with British security."
Defence Secretary Michael Fallon told ITV News Mosul would soon be 'encircled' to expel militants from the city.Read the full story ›
Michael Fallon is to stay on as Defence Secretary in Theresa May's new government, Downing Street has announced.
Britain is not turning its back on the world, the defence secretary has said after Britain voted to the leave the EU.
Michael Fallon told ITV News: "I'll be speaking to all our major allies this afternoon to reassure them that Britain is not turning its back on the world."
Britain leaving the EU would weaken security in the West, Defence Minister Michael Fallon has said.
Speaking at NATO, Mr Fallon said: "This is a dangerous moment. No country has ever left either NATO or the European Union and that would clearly weaken the security of Western Europe. Those are the twin pillars of our security."
He also dismissed reports of a possible EU army, saying Britain would veto such a proposal.
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The Government remains committed to building parliamentary support for RAF air strikes against Islamic State (IS) in Syria despite Russia's intervention in the conflict, Defence Secretary Michael Fallon has said.
Following talks in London with US defence secretary Ash Carter, Mr Fallon said Russian strikes in support of President Bashar Assad risked making "a grave situation even worse".
However he made clear that ministers would not be deterred from seeking support for a new Commons vote for British warplanes to join the US and other coalition aircraft in attacking IS - also referred to as Isil - in Syria as well as Iraq.
We will continue to build the case for military action in north east Syria where Isil is headquartered, where its command and control is. Russian intervention doesn't change that. We will continue to build the case for intervention in the new parliament.
US Defense Secretary Ash Carter has said Russia's actions in Syria are "risky" and "will only inflame the civil war".
He added that Nato "stood strong" against Russia in Ukraine and this "strengthened posture".
ITV News Correspondent Dan Rivers is at the news conference in London.
Carter: NATO stood behind Turkey on Russian incursion Fallon: reinforced determination to protect collective security
Carter: way out of tragedy is for political transition. For Assad to end his rule. Russia best placed to aid that transition
Defence Secretary Michael Fallon has said Russian behaviour "threatens our allies".
He also announced £70m is to be invested in the next five years to drive innovation in new technology for the military.
He is speaking at a press conference in London along with US Defence Secretary Ash Carter.
Mr Fallon said: "Aggressive Russian behaviour continues threatening our allies in Europe."
ITV News Correspondent Dan Rivers is at the news conference.
Fallon: aggressive Russian behaviour continues threatening our allies in Europe.
Fallon: been concerned for some time about build up of Russian forces. Another reminder to NATO to look to collective security as a whole