Labour and the Conservatives have renewed their pledge to spend 2% of GDP on defence if they win the General Election, meeting NATO's target.
Mrs May insisted she would ensure "brave" troops were given the cash they needed and claimed Labour's Jeremy Corbyn would put defences "at risk".
Shadow defence secretary Nia Griffith said Labour was committed to the 2% target and added: "The Tories' hypocrisy on defence knows no bounds.
"Their cuts have left our forces more under-resourced and underpaid than at any time in the modern era," she said.
Professor Malcolm Chalmers, deputy director of defence think tank the Royal United Services Institute called the extension of the annual commitment "significant".
"It protects defence to some extent from the uncertainty about the impact of Brexit on government finances overall and the possibility that a new spending review might require further cuts," he added.
Though Professor Chalmers said an annual increase "will not be enough to avoid some hard choices on defence priorities".
It follows claims by a number of retired senior military figures that the Ministry of Defence (MoD) was guilty of "accounting deception" in meeting the 2% spending target set by NATO.
A letter signed by retired admirals, generals and air marshals, as well as wounded service-personnel and leading academics claimed that the MOD quoted "irrelevant financial statistics."
It added: "Government boasts of spending 2% of GDP on defence are widely criticised as an accounting deception."
However in February a NATO official said: "According to our calculations, five allies, including the United Kingdom, currently meet or exceed Nato's 2% spending benchmark for 2016."
Theresa May said: "As Prime Minister I always have and always will put Britain's national security first.
"That is why if elected on June 8 I will ensure that the UK continues to spend at least 2% of GDP on defence and that the budget rises every year," she added.
The UK has previously called upon fellow NATO countries to increase their spending on defence.