Michael Clarke, the former director-general of the Royal United Services Institute (RUSI), told MailOnline that the army must be better at driving recruitment.
UK troops have trained some 30,000 Ukrainians, including civilians, on British soil under Operation Interflex, the multinational support mission backing Ukraine in its fight against Russia.
Veteran Ben McBean (pictured) - who lost an arm and a leg in Afghanistan - has said Britons would have to be dragged kicking and screaming to enlist if they were called to fight Russia.