A former Commander of Joint Forces Command says the regular Army is too small and "isn't going to cut it" in the face of the Russian threat.
General Sir Richard Barrons says the current regular Army is a fraction of the size it needs to be.
He also believes Ukraine could still win in the war with Russia, as long as the West does not give up.
General Sir Richard Barrons said: "Everyone will recognise the regular Army as very small, it's half the size of the Army I joined in the 1980s, in the Cold War.
"So, put that in context, a regular Army of about 76,000 people now, really, probably has the equipment for about 40,000 in the field at any one time, probably has stocks for about a month of hard fighting, so none of that is enough."
He added: "That would mean if you put the entire British Army into Ukraine you might have made it responsible for about a third of the fight for the Donbas, realistically, and everything else would have been done by allies.
"So this isn't going to cut it."
Defence Secretary Ben Wallace has reportedly written to Prime Minister Boris Johnson calling for a 20% increase in defence spending a year by 2028, to meet shortfalls in military capabilities.
That would take defence spending to 2.5% of GDP.
"I think it does sound about right," General Sir Richard Barrons said.
"To put that into perspective, .5% or half a per cent of GDP is about £11bn a year and the current defence budget is about £46bn in the year of which only £14.5bn is spent on capital investment.
"It's not that we can't afford to find £11bn a year from a public sector of well over £850bn.
"It's just it will be at the expense of something else we really care about. So, a really tough decision, but also unavoidable."
General Barrons believes the fight for Ukraine is the Cold War moment that never came, and despite Russia's apparent progress in the Donbas, if Britain and the rest of NATO support their ally in the right ways, Ukraine can still win.