General Sir Richard Shirreff, who served as NATO's Deputy Supreme Allied Commander in Europe between 2011 and 2014, said that an attack on Estonia, Lithuania or Latvia – all NATO members – was a serious possibility and that the West should act now to avert "potential catastrophe," The Independent reports.
He has written a fictional book 2017: War with Russia, but told BBC Radio 4's Today programme the events it describes were "entirely plausible."
General Shirreff said: "The chilling fact is that because Russia hardwires nuclear thinking and capability to every aspect of their defense capability, this would be nuclear war."
"We need to judge President Putin by his deeds not his words," he added. "He has invaded Georgia, he has invaded the Crimea, he has invaded Ukraine. He has used force and got away with it.
"In a period of tension, an attack on the Baltic states… is entirely plausible."
Read alsoFT: NATO rapid unit not fit for eastern Europe deployment, say generalsNATO members would be obliged under Article 5 of its founding treaty to come to the defense of another member if it came under attack.
General Shirreff said that Mr. Putin could be persuaded into an intervention in the Baltic by a "perception" of weakness in NATO, and predicted that, as in Crimea, the Russian president would present his actions as an act of defense to protect the large Russian-speaking minorities in those countries.
NATO has already stepped up defenses in the Baltic states, but General Shirreff said that it needed to "raise the bar sufficiently high for any aggressor to say it is not worth the risk."
"I would argue the bar is not high enough at the moment," he added.
In the preface to his book, General Shirreff is critical of recent defense cuts in the UK, writing: "A country famous for once 'walking softly and carrying a big stick'…now had a leadership that shouted loudly but, thanks to ongoing defense cuts, carried an increasingly tiny and impotent stick."