Russia will attack the West if provoked in Syria, the UK's former ambassador in Moscow said, as he warned of "the most dangerous situation in our relations I have ever seen."
It comes with relations between the West and Russia at their worst point since the Cold War.
US Presidential frontrunner Hillary Clinton has backed imposing a "no fly zone" in Syria, where Russia is accused of deliberately bombing civilians, while Russian President Vladimir Putin warned of "problems" if the US continued its "confrontation" with Russia.
Sir Anthony Brenton, who served as Britain's ambassador to Russia from 2004 to 2008, said that escalating tensions could result in open conflict with Russia.
"They are behaving as toughly and aggressively as I have ever seen them," Brenton wrote in the Daily Mirror.
"If we act on Syria in any way that could escalate to a Russian attack, they are signalling as clearly as they can that they would retaliate."
In recent weeks the US and UK have fiercely condemned Russia's bombing campaign in Syria, accusing Russian forces of committing war crimes and deliberately targeting schools, hospitals, and an aid convoy. Moscow denied the claims.
Russia recently revealed that it has deployed nuclear capable missiles to its Baltic enclave in Kaliningrad, and completed a week long nuclear war exercise reportedly involving 40 million civilians. It has also cancelled a weapons grade plutonium agreement with the US, and unveiled the new Satan II intercontinental ballistic missile, capable of obliterating an area the size of France.
In response Nato has deployed 4,000 troops in eastern Europe and the Baltics.
Brenton, who has dealt with Russia for more than 20 years in the Foreign Office, said that though Russia "means business" it is "weak" and called for a diplomatic resolution to the tensions.
"They do not want war. They would lose. These shows of strength are not an intention to fight, but a signal that fighting must be avoided," he said.
On Thursday, Putin dismissed claims that Russia posed an increased threat, declaring "Russia doesn't plan to attack anyone."
Brenton described Putin as "hard-headed, well-briefed and ruthless," and warned that "for Russia backing down is now close to impossible. The loss of face would almost certainly be the end of Putin."