The Foreign Secretary says Britain must "decisively" raise defence spending once the country has left the EU.
In a speech in London, Jeremy Hunt said it would demonstrate the UK's commitment to protecting democratic values around the world.
He said, instead of "plugging gaps", there needs to be more money for things like cyber defence and artificial intelligence.
At the same time, he warned it was not sustainable to expect the US to carry on spending 4% of its GDP on defence, while other NATO allies spent only 1% to 2%.
Mr. Hunt said it was key not to leave our "great ally", the United States, and stand for the defence of democratic values.
The Foreign Secretary is widely expected to be a contender for the Tory leadership when Theresa May steps down, and his address will be seen as a pitch to Conservative MPs concerned about the level of defence spending.
He acknowledged the UK currently accounted for almost 20% of total EU defence spending, and that British forces possessed a "hugely disproportionate share" of some key capabilities such as heavy lift transport aircraft.
But at a time of evolving threats, he said the country needed to be prepared to do more to defend its traditional values.
"We are in a multipolar world without the assurance provided by unquestioned American dominance," he said.
"We face a more aggressive Russia and a more assertive China.
"We simply do not know what the balance of power in the world will be in 25 years time.
"The new domains of space and cyber and the immense capabilities of artificial intelligence will transform the conduct of warfare."
Mr Hunt's comments were dismissed by Labour's shadow defence secretary, Nia Griffith.
She wrote on Twitter: "Jeremy Hunt has been a Cabinet Minister since 2010. Since then the Tories have cut defence spending by £9 billion in real terms.
"If he was so bothered, you'd have thought he might have said something a bit sooner?"
Mr Hunt, who backed Remain in the referendum but has since sought to appeal to Brexiteers, said it was essential to honour the 2016 vote by leaving the EU "cleanly and properly".