General Mike Jackson, who was Chief of the General Staff from 2003 to 2006, said the Army’s armoured corps was a "shadow" of what it was a few decades ago, according to Daily Mail.
He said the 80,000-strong Army would now struggle to fight a battle in the way it did in the past.
The Ministry of Defence is under pressure to deliver military capability for less money and also to modernise the force to meet diverse threats – but it has dismissed claims that Army numbers could be cut.
Speaking to ABF The Soldiers’ Charity for its podcast General Talk, Mike said, ‘When I joined, the Cold War was very cold. The whole strategic posture was deterrence of the then Soviet Union and the Warsaw Pact. It needed mass. We had a comparatively large army for peace time.’
But he said it has shrunk since.
"It worries me that 80,000 may not be big enough," he added.
The former Para said the Army could fight a conventional battle "at a pinch", but "it would take some preparation time".
Mike said, "We are down to the position where really if we get it right we can field a single division. Perhaps of two or three brigades. That is [the] maximum effort we could expect of today’s Army."
A brigade has around 6,500 troops, while a divison typically has between 8,000 and 25,000 personnel.
When Mike joined the Paras in 1970 the Army’s regular strength was 176,000, with 80,000 reservists.
He added, "The Royal Armoured Corps is pretty much a shadow of what it was when I joined."
The MoD’s review could see soldiers moved into other areas, for example cyber, to match the threat and skills needed as the world changes, sources said.
A report by the Royal United Services Institute think-tank last year warned that British troops would be "comprehensively outgunned" in a war with Russia due to a "critical shortage" of artillery.