The Liberal Democrats' foreign affairs spokesman Chuka Umunna has vowed to spend nearly £1 billion more on defence than the Conservatives over the next six years.
Mr Umunna said the economy would grow faster if Brexit was scrapped, meaning the UK's spending of 2% GDP on defence would also go further.
Speaking in Watford on Monday morning, Mr Umunna said the Liberal Democrats were committed to NATO which he called the "cornerstone of the defence of our country".
"We must provide credible deterrents that convince others that NATO is committed to Europe’s collective defence," he said.
"This is why Liberal Democrats will ensure the UK upholds our NATO responsibilities, including by spending 2% of GDP on defence.
"Under Liberal Democrats in government, this 2% will be worth £993 million more in 2024-25 than under a Tory government because, using the Remain bonus, we will enhance the UK’s ability to play our part in NATO and maintain our security.
"In so doing, we will ensure the men and women in our Armed Forces have the support and resources they need to do their jobs."
The former Labour MP also said re-electing the Conservatives risks being a "huge backward step" in terms of security.
"Re-electing a government that aligns itself with the forces ranged against the liberal rule-based order, forces which undermine NATO, cannot be the answer," he said.
Meanwhile, the Conservatives launched its party manifesto over the weekend.
Leader Boris Johnson said his party will "increase funding" for the military and that they will not cut the Armed Forces "in any form".
During a speech in Telford, Shropshire, he said defence spending would remain at 2% GDP and promised to increase funding to the Armed Forces each year by 0.5% above inflation.
The Liberal Democrats also released its manifesto last week, which includes plans to stop Brexit and also improve the quality of housing for service personnel.
The party wants to give military tenants the same legal rights regarding repairs and maintenance as private tenants.