Britons will be warned that that they could be called up to fight if the war breaks out in Russia.
Chief of the general staff General Sir Patrick Sanders is set to stress the need for ministers to "mobilise the nation" in the event of a wider conflict against Russia amid its invasion of Ukraine.
Sanders added that the military is "too small" to handle the conflict on its own.
However, it has been reported that General Sr Patrick not thought to be in favour of conscription.
Chief of the general staff General Sir Patrick Sanders is calling on Britons to change their mindset
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It comes as Nato bosses have warned allied nations to be prepared to take any and all actions necessary to fight Vladimir Putin's forces.
In an assessment of the armed forces' capabilities, Sanders said: "Our procurement record has been poor and our land industrial base has withered. Furthermore our Army Reserve is not as capable and credible as we need it to be."
The Army head is standing down in six months after allegedly falling out with Chief of the Defence Staff Admiral Sir Tony Radakin.
Sanders has previously been openly critical of the Army's equipment and its troop numbers. He will make his plea at the International Armoured Vehicles Conference in Twickenham later today, according to the Daily Telegraph.
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Queen Camilla with General Sir Patrick Sanders
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Sanders' impending departure was mourned by then-chair of the Commons defence select committee Tobias Ellwood.
Ellwood commended him for being "willing to tell political leaders what they should hear rather than what they'd like to hear".
Sanders' warning comes as a senior Nato official warned that the West faces all-out war with Moscow within the next two decades. It is one of several warnings to be given by military top brass and ministers in several member states.
German defence minister Boris Pistorius said: "We have to take into account that Vladimir Putin will one day even attack a Nato country. Our experts expect a period of five to eight years in which this could be possible. In the moment I don’t think a Russian attack is likely."
On Tuesday, Nato signed a contract worth almost £1billion to produce 220,000 artillery rounds for member countries, paving the way for allied nations to backfill their arsenals and provide Ukraine with further ammunition.
It is also mobilising some 90,000 troops for war games aimed at sending a message to Russia not to consider attacking any member countries.
Chair of the Nato military committee Admiral Rob Bauer said: "We have to realise it's not a given that we are in peace. And that's why we (Nato forces) are preparing for a conflict with Russia.
"But the discussion is much wider. It is also the industrial base and also the people that have to understand they play a role."