The Conservatives' new commitment to increase defence spending by at least 0.5% above inflation every year until 2022 was overshadowed by the leak of Labour's draft general election manifesto on Wednesday evening (11 May).
Prime Minister Theresa May will make the pledge just a day after meeting Jens Stoltenberg, the secretary-general of the North Atlantic Treaty Organisation (NATO).
The Tory premier will also confirm that her government would continue to meet the military alliance's defence spending target at 2% of GDP per year.
"Under my leadership the Conservatives will ensure that the brave men and women of our Armed Forces have the equipment and resources they need to keep our country safe – and that we meet our obligations to the world," May will say.
"But a strong military is only possible with a strong economy, and Jeremy Corbyn would put both at risk if he makes it to Downing Street propped up by the Lib Dems and SNP in a coalition of chaos.
"Only a vote for me and my Conservative team will deliver the strong and stable leadership that is vital for our national security."
Conservative sources said the commitment was in "stark contrast to the nonsensical policies" of Jeremy Corbyn. The leaked draft copy of Labour's manifesto sees the party backing Trident, the UK's nuclear deterrent, but the opposition party is proposing a defence review. Shadow Chancellor John McDonnell has also previously called for a Ministry of Peace, rather than defence, to be established.
But Shadow Defence Secretary Nia Griffith hit back at the Conservatives and claimed that the British Armed Forces have been left "under-resourced and underpaid".
"The severe cuts imposed on the defence budget since 2010 have seen the Army shrink to 78,000, its smallest size since the Napoleonic Wars and far short of the last Tory manifesto pledge to keep it above 82,000. And countless knee-jerk decisions have weakened Britain's defence capabilities, including scrapping our Nimrod maritime patrol aircraft, leaving us reliant on allies to track Russian submarines off the British coast," she said.
"Labour is committed to spending at least 2% of GDP on defence and ensuring that our Armed Forces have the necessary capabilities to fulfil the full range of our NATO obligations, and we will continue to press other members of the Alliance to do the same."
May's pledge comes less than a month before the general election on 8 June. The latest opinion poll from Panelbase, of more than 1,000 people between 5 and 9 May, gave the Tories a 17 point lead over Labour (48% versus 31%).