The Conservative Armed Forces Minister Mike Penning has accused Jeremy Corbyn of "collaborating with Russia" in response to comments that British troops about to be deployed to Estonia were there to "escalate tensions" between Russia and NATO.
Mr Penning said the comments made on Wednesday by a spokesperson for Mr Corbyn showed that Labour "cannot be trusted with Britain's national security".
800 UK troops will be part of a Nato taskforce that will be stationed in Estonia this spring, amid fears of Russian aggression in the region, and concerns President elect Donald Trump may not honour the United States's commitments as a member of Nato to defend other members.
In an interview with BBC Wales today, Mr Corbyn said it was "unfortunate that troops have gone up to the border on both sides".
He added: "I don't want to see any more troops deployed on the borders between Nato and Russia, I want to see a de-escalation, ultimately a de-militarisation and better relationships between both sides of it... there cannot be a return to a Cold War mentality."
But Mr Penning said: "Britain has Nato's second biggest defence budget and plays a leading role in the alliance. It is unprecedented for a leader of the opposition to attack the defensive deployment of British troops in Nato territory.
"These comments suggest that the Labour leader would rather collaborate with Russian aggression than mutually support Britain's Nato allies. As with Trident, everything Labour says and does shows that they cannot be trusted with Britain's national security."
