Prime Minister Sir Keir Starmer and Defence Secretary John Healey were told Russia has the "largest and most battle-hardened forces in the world".
But Britain is "cutting her combat power and readiness for war", one expert said.
Reductions to the Ministry of Defence are getting "worse under Labour", making Britain an outsider among Nato allies, critics declared.
Patrick Mercer, a former Army colonel and MP, said: "As the war developed, Boris Johnson bellowed jingoistically whilst, towards the end of Tory rule, the Chief of the General Staff told the Army to prepare for a war in Europe.
"Then came the real perfidy ongoing defence cuts weren't reversed, they were increased, with a further 10,000 combat troops being scrapped.
"Starkly, this happened as Nato countries like Poland began to expand her forces threefold, doubled her defence spending and fully mobilised her population and industry.
"Worse was to follow under Labour. Earlier this month, John Healey held his hands high and said that Britain's forces are not 'ready to fight'.
"Then, as if to guarantee that statement, a few days ago further, swingeing defence cuts were announced right in the middle of a major escalation in Ukraine.
Dithering
"A thousand days' warfare have given Russia the largest and most battle-hardened forces in the world. They must must be taken seriously.
"Yet, alone in Nato, Britain is cutting her combat power and readiness for war.
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