Britain should "train and equip" a "citizen army" to ready the country for a potential land war, the head of the Army has said.
But General Sir Patrick Sanders, the outgoing Chief of the General Staff (CGS), said even that would be "not enough" as he pointed to allies in eastern and northern Europe "laying the foundations for national mobilisation".
In a speech on Wednesday, the military top brass 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 Russia.
It comes after Defence Secretary Grant Shapps in a speech last week said the world is "moving from a post-war to pre-war world" and the UK must ensure its "entire defence ecosystem is ready" to defend its homeland.
Wednesday's comments by Sir Patrick, made during a speech at the International Armoured Vehicles conference in west London, do not mark the first time he has pushed for greater readiness of Britain's armed forces for conflict.
Speaking in 2022 in the months after Russian president Vladimir Putin's invasion of Ukraine, the Army chief said "this is our 1937 moment" - a reference to preparations made for the start of the Second World War - and that the British Army should be at "high readiness".
In his speech on Wednesday, Sir Patrick said Britain could not rely on its navy and air power, arguing "we must be able to credibly fight and win wars on land".
He said: "We need an Army designed to expand rapidly to enable the first echelon, resource the second echelon and train and equip the citizen army that must follow.
"Within the next three years, it must be credible to talk of a British Army of 120,000, folding in our reserve and strategic reserve. But this is not enough.
"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, preparing Sweden for entry to Nato, taking preparatory steps to enable placing our societies on a war footing when needed are now not merely desirable but essential.
"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."
But Rishi Sunak’s government slapped down the suggestion from Britain’s army chief that the public could be called up to fight a war against Russia.
Asked if Mr Sunak agreed with the possible move, the Prime Minister’s official spokesman said: “No.”
Pressed whether he ruled out conscription following the idea floated by the top officer of public call-ups to the military, the spokesman added: “There is no suggestion.
“The Government has no intention to follow through with that.
“The British military has a proud tradition of being voluntary.”
Asked if the PM believed the British military was strong enough to fight against Russia, he added: “These kind of hypothetical scenarios, talking about conflict, are not helpful.”
He declined to get drawn into discussions about “hypothetical wars”.
Sir Patrick also made a pitch for more backing for the Army, saying those serving "rightly want the equipment that enables them to deliver lethal capability".
He has been a vocal critic of cuts to troop numbers and military spending.
He 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.
Chief of the Defence Staff, Admiral Sir Tony Radakin, denied the claim when asked by MPs about the reports in July.
Sir Patrick has not been alone in warning against allowing the Army to be shrunk further.
Last week, a former chief of the general staff of the British Army said the UK risks a repeat of the 1930s unless more is invested in its armed forces.
General Lord Dannatt hit out at the shrinking size of the army, which he said has fallen from 102,000 in 2006 to 74,000 today and is 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 is "a serious danger of history repeating itself".
Mr Shapps has insisted the size of the Army will not dip below 73,000 under the Conservatives.
He told Sky News on Sunday: "It's not projected to go down to 50,000.
"It is actually, specifically, to 73,000 plus the reserves."
