Sir John Major has warned not funding defence is a "betrayal" of Britain's future in the latest blow to Sir Keir Starmer.
He joined calls by a series of former defence chiefs and MPs who have urged the Prime Minister to bolster Britain's defence budget.
In a letter to The Times, the former Prime Minister said: "We are living in a belligerent environment, which may worsen. The UK — and Europe more generally - must be sufficiently equipped and able to defend itself.
"The argument that 'we cannot afford it' will be irrelevant if Putin succeeds in Ukraine — and then, perhaps, moves on into Nato territory.
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"If we and our European allies are not prepared to make the necessary sacrifices to protect our people and our sovereign territory, it will be a betrayal of our history and our future."
In another letter, General Lord Richard Dannatt, the former chief of the general staff, compared the current defence funding situation to Britain before the Second World War.
He said: "In 1939, when war broke out, the figure shot up to 19% and in 1940 when we were fighting for our very survival it rose to the staggering figure of 46%.
"That is the frightening cost of fighting a war that for a modest increase in defence spending earlier could have been avoided.
Sir John Major said not funding defence would be a 'betrayal of our history and our future'
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"Putin is not just a threat but a proven aggressor. Today, where is the prudent Chamberlain who began the rearmament programme or the Churchill who led the country in its darkest hour? No-one of that calibre is at present resident in Downing Street."
He echoed similar remarks as Donald Trump, who previously said the Prime Minister was "no Winston Churchill".
A third letter, penned by an All-Party Parliamentary Group (APPG) said they agreed with Lord George Robertson who accused British leadership of "corrosive complacency".
But they warned defence funding should not be used "for point-scoring between political parties", citing this as the reason for a new APPG.
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Rachel Reeves has outlined a limited funding boost to defence
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They said: "We have formed a new group, the all-party parliamentary group on rearmament, to raise public awareness about the threats we face; emphasise the reduced state of our military capability; and create a political consensus that the government must rearm our nation.
"The government called for a national conversation on defence in the strategic defence review last year. We are starting it."
Chairing the 50 person group is Dr Mike Martin, a Liberal Democrat MP.
The letter was signed by the APPG's vice-chairs, Field Marshal Lord Richards of Herstmonceux, Conservative MP Lincoln Jopp who is also a former commanding officer in the military, and Labour backbencher Tim Roca.
Dr Mike Martin will chair an APPG to call for increases to the defence budget
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Rachel Reeves has outlined a limited boost in defence spending of less than £10billion over four years, not enough to fill the £28billion budget hole the Ministry of Defence (MoD) currently faces.
She has been accused of "failing to prioritise national security" after she reportedly told officials she did not want to give money to a department that did not have "gender parity".
Continued delays to the Defence Investment Plan have resulted in small and medium-size enterprises unable to scale up production in emerging technologies such as drones.
The first newsletter from the APPG for rearmament pointed out Russia's Northern Fleet has 22 submarines, more than double Britain's fleet.