Ben Wallace made the candid comments suggesting Putin had lost his mind while also comparing the Russian leader to Tsar Nicholas I, who struggled for allies during the Crimean War in the mid-19th century.
"We've got a busy adversary now in Putin, who has gone full tonto," Wallace -- a former army officer -- told serving personnel in a government building in Westminster, Britain's Press Association news agency reported.
"Tsar Nicholas I made the same mistake Putin did... he had no friends, no alliances.
"The Scots Guards kicked the backside of Tsar Nicholas I in 1853 in Crimea -- we can always do it again," Wallace, who served in the same regiment, was overheard saying.
The minister's unvarnished assessment came after Prime Minister Boris Johnson on Tuesday said Putin was in an "illogical and irrational frame of mind".
Asked about Wallace's reported assessment, Johnson's official spokesman told reporters: "The defence secretary is more astute to make that judgment than I."
Foreign Secretary Liz Truss meanwhile said Putin was "highly likely" to launch a full-scale invasion of Ukraine and attack Kyiv.
Britain and the US have repeatedly cited intelligence as indicating that Moscow is planning such a move.
However, Truss noted London does not yet have "the full evidence" that Russian troops have crossed into Ukrainian territory, including rebel-held areas, calling the current situation "ambiguous".
'Defensive weapons'
Meanwhile in parliament, Johnson confirmed Britain would send further military supplies to Ukraine "in light of the increasingly threatening behaviour" from Russia.
"This will include lethal aid in the form of defensive weapons and non-lethal aid," he told MPs.
The UK last month deployed some 2,000 anti-tank weapons to Kyiv along with military trainers -- who have since left the country -- as Western nations stepped up their support for Ukraine.
London is ready to guarantee up to $500 million (£368 million) in loans to Kyiv to promote economic stability and reforms, the foreign office said ahead of Johnson's comments.
In December, it increased the amount of financial support available to Ukraine to £3.5 billion and signed a treaty on modernising its navy.
Earlier this month it also announced £100 million in extra assistance to be provided over three years to help the ex-Soviet country boost the economy and reduce dependency on energy imports.
The latest commitments come a day after Britain slapped sanctions on five Russian banks and three billionaires, in what Johnson branded "the first barrage" of measures in response to the Kremlin's actions.
However, he faced criticism from numerous lawmakers, including from within his ruling Conservatives, that the measures were woefully insufficient.
He and his ministers have insisted tougher measures are set to follow but depend on Moscow's actions.
Johnson also announced Wednesday that his culture minister had asked media regulator Ofcom to review the UK broadcasting licence of Kremlin-backed television channel RT.
In a leaked letter to Ofcom, Culture Secretary Nadine Dorries urged the agency to take "timely and transparent" action against RT, which she warned seeks to spread "harmful disinformation".
An Ofcom spokesperson confirmed receipt of the letter to AFP, adding: "All licensees must observe Ofcom's rules, including due accuracy and due impartiality.
"If broadcasters break those rules, we will not hesitate to step in. Given the seriousness of the Ukraine crisis, we will examine complaints about any broadcaster's news coverage of this issue as a priority."
Russian foreign ministry spokeswoman Maria Zakharova hit back on Telegram saying “If Britain turns its threat towards Russian media into a reality, retaliatory measures will not take long to come.
"British journalists can ask their German colleagues what this looks like," she said.
German broadcaster Deutsche Welle closed its Moscow bureau at the start of this month after Russia shut the outlet's local operations to punish Germany for banning a service of a Russian state TV network.
