Commons Defence Committee chairman Tobias Ellwood has asked Boris Johnson whether he would support increasing the defence budget to "at least 3%" of gross domestic product (GDP) to deal with rising complex global threats.
Appearing in front of the Liaison Committee of senior MPs, the Prime Minister said he had already increased the budget as part of the Integrated Review.
He told the Liaison Committee: "I wish to be clear that the 2.2% is an increase.
"It is not that we are remaining at 2.2%, it has gone up to 2.2% as a result of the biggest investment in our armed forces in defence since the Cold War – £24 billion of investment."
Mr Johnson told the group of senior MPs the Review is a "full spectrum investment".
"It covers everything from cyber to modernising and protecting our tanks, to enabling us to go ahead with the future combat air system that I think will be essential for defending our skies in the future and enabling us to project force" he said.
"The UK is one of the few countries in the world able to project force at 8,000 miles or more – and this integrated defence, security and foreign policy review allows us to continue to do just that."
Boris Johnson’s appearance in front of the Liaison Committee comes the same day as a challenge by Labour leader Sir Keir Starmer to allow MPs to vote on a "cut" to the UK military.
In addition to the Army's regular force being reduced, the Defence Command Paper revealed the service's structure is being reorganised and detailed changes in equipment.
Nine Royal Air Force Reapers are being replaced by Protector in the next three years, while three E-7 Wedgetails will take over from the outgoing E-3 Sentry, with tranche one Typhoons being retired by 2025.
The C-130J Hercules aircraft, meanwhile, is being retired in 2023, with the capability transitioning to the C-17 and A400M.
New automated minehunting systems will also be deployed in the future, replacing the Sandown and Hunt classes, which will both retire in the 2020s.
Watch: Defence Secretary Ben Wallace outlined the future plans for the military earlier this week.
Despite the Prime Minister setting out a £16.5 billion increase to the MOD's budget over the next four years in November, the British military is to make £1 billion of cuts over this year in response to a defence funding black hole.
The move is part of cost-cutting measures implemented by the Ministry of Defence (MOD) to make in-year savings on a £13 billion gap in funding, evidenced by a National Audit Office (NAO) report.
The US remains the alliance's biggest contributor, spending an estimated 3.73%.
With NATO estimating Britain's personnel numbers for 2020 at around 156,200, it showed the UK has seen an increase in serving personnel compared to the previous year's figure of 144,400.
However, despite the increase of more than 10,000, the UK does not feature in the top five Alliance countries, according to the latest statistics.
The United States leads the way, with 1,346,000 servicemen and women, followed by Turkey (437,200), France (208,000), Germany (186,900) and Italy (175,500).