The West should prepare for a worst-case scenario war with Russia as Vladimir Putin "respects strength", an ex-Nato chief warned.
Sir Richard Shirreff, a former Nato deputy strategic commander for Europe, said the security bloc must prepare for a clash with Moscow.
It comes after the UK defence secretary Ben Wallace warned Vladimir Putin could declare a "world war" against supposed "Nazis" in the coming days.
Sir Richard supported Mr Wallace in his call for the West to "double down" on its support for Ukraine.
This morning, the ex Nato chief told BBC Radio 4's The Today programme: "The worst case is war with Russia.
"By gearing itself up for the worst case, it is most likely to deter Putin because ultimately Putin respects strength."
Sir Richard previously told LBC : "Number one, it (Nato) has got to be preparing itself for the worst case in order to take risk, which it is doing... but I think it probably needs to be able to take more risk.
"Ultimately, Nato needs to think about the position of Ukraine and how best to guarantee Ukraine's security in the longer term, and these are some pretty profound implications for the alliance."
He added that Nato must realise the conflict could escalate into a world war and the bloc should "change its mindset to ensure it is ready for the worst case".
"Frankly a deployment of 40,000 troops of the Nato response force is a good start but it's not what needs to be done," Sir Richard said.
"There needs to be a fundamental reappraisal of strategy, defence capability, mobilisation, rearmament, regeneration of armed forces across the Nato alliance."
It comes after nuclear-capable Kalibr missiles were seen being loaded onto a Russian submarine in chilling photographs.
The Veliky Novgorod submarine was in a dock in Sevastopol, Crimea, according to Ukraine Now Telegram channel.
It said: "Russian Kalibr-type cruise missiles were loaded onto the Veliky Novgorod diesel-electric submarine - the very ones bombing the peaceful cities of Ukraine."
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Russian propaganda has now shown submarines in the Black Sea firing a barrage of Kalibr missiles at Ukraine.
It is reportedly the first time Russian submarines have been used to attack Ukraine.
Meanwhile, Mr Wallace confirmed 15,000 Russian troops had been killed in Ukraine, according to Britain's latest estimates.
He added that to date the UK has provided more than 5,000 anti tank missiles, five air defence systems with more than 100 missiles, 1,360 anti-structural munitions, and 4.5 tonnes of plastic explosives.
Britain has also sent over 90,000 ration packs, 10 pallets of medical equipment, 3,000 pieces of body armour, 77,000 helmets and 3,000 pairs of boots, he said.
Mr Wallace told the House of Commons: “All of Europe can now see the true face of President Putin and his inner circle.
“His intention is only to destroy, to crush, to rub out the free peoples of Ukraine. He does not want to preserve. He must not be allowed to prevail."
No10 earlier stressed Britain's objective was not regime change in Russia.
