British citizens should be “trained and equipped” to fight in a potential war with Russia – as Moscow plans on “defeating our system and way of life”, the head of the British Army has said.

General Sir Patrick Sanders, the outgoing Chief of the General Staff (CGS), said increasing army numbers in preparation for a potential conflict would need to be a “whole-of-nation undertaking”.

The comments, first reported by the Daily Telegraph, are being read as a warning that British men and women should be ready for a call-up to the armed forces if NATO goes to war with Vladimir Putin.

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It comes after Defence Secretary Grant Shapps said in a speech last week that we are “moving from a post-war to pre-war world” and the UK must ensure its “entire defence ecosystem is ready” to defend its homeland.

But Downing Street ruled out any move towards a conscription model, saying that army service would remain voluntary.

Sir Patrick has been a vocal critic of cuts to troop numbers and military spending.

In his speech at the International Armoured Vehicles conference in west London, he said the UK must urgently expand the size of the army to around 120,000 within three years – up from around 74,000 now.

But he said “this is not enough” and training and equipping a “citizen army” must follow.

He pointed to this happening across Europe, telling the audience: “Our friends in eastern and northern Europe, who feel the proximity of the Russian threat more acutely, are already acting prudently, laying the foundations for national mobilisation.

“As the chairman of the NATO military committee warned just last week, and as the Swedish government has done…taking preparatory steps to enable placing our societies on a war footing when needed are now not merely desirable but essential.”

Sir Patrick added: “We will not be immune and as the pre-war generation we must similarly prepare – and that is a whole-of-nation undertaking.

“Ukraine brutally illustrates that regular armies start wars; citizen armies win them.”

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Sir Patrick added that Ukraine was currently the “principal pressure point on a fragile world order that our enemies wish to dismantle”.

He continued: “(The war in Ukraine) is not merely about the black soil of the Donbas, nor the re-establishment of a Russian empire, it’s about defeating our system and way of life politically, psychologically, and symbolically.

“How we respond as the pre-war generation will reverberate through history. Ukrainian bravery is buying time, for now.”

Sir Patrick also said that our predecessors “stumbled into the most ghastly of wars” after failing to “perceive the implications of the so-called July Crisis in 1914”, referring to a series of diplomatic and military escalations leading to the outbreak of the First World War.

“We cannot afford to make the same mistake today,” he added.

Sir Patrick will be replaced as CGS in June by General Sir Roly Walker, an announcement that followed reports he was being forced out in response to his outspoken comments.

Tobias Ellwood, a former defence minister who has served alongside Sir Patrick, said the military chief should be “listened to carefully”.

“What’s coming over the horizon should shock us. It should worry us and we are not prepared,” he told Sky News.

The MP for Bournemouth East said that following decades of post-Cold War peace, there was a growing sense authoritarian states could “exploit our timidity, perhaps our reluctance to really put fires out” – pointing to Russia’s invasion of Ukraine.

“So Patrick Sanders is saying prepare for what’s coming over the horizon – there is a 1939 feel to the world right now,” he said.

“These authoritarian states are rearming. There’s a risk averseness about the West in wanting to deal with that and our global institutions such as the United Nations aren’t able to hold these errant nations to account.”

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Mr Ellwood went on to say the army was “overstretched”, in part because of issues to do with pay and accommodation.

He said the army, as well as the navy, was about “half the size of what it should be” while the RAF was lacking the equipment it needs.

Warnings about the “shrinking size” of the army have also been sounded by former military chief General Lord Dannatt, who told The Times numbers had reduced from 102,000 in 2006 to 74,000 today and were still “falling fast”.

He drew parallels with the 1930s when the “woeful” state of the UK’s armed forces failed to deter Adolf Hitler, saying there was “a serious danger of history repeating itself”.

Speaking to Sky News about the comments, Mr Shapps insisted the size of the army would not dip below 73,000 under the Conservatives’ watch – even as he resisted Lord Dannatt’s calls to up the defence budget.

The government is currently spending around 2% of GDP on defence, but some want to see it rise to 3%.

The government’s target is 2.5%, but Mr Shapps told Sky News on Sunday that “we’re not there yet”.