The UK should consider "decisively" increasing defence spending after Brexit, Foreign Secretary Jeremy Hunt has said.
He told the Lord Mayor's Banquet in London the threats facing the UK had changed "markedly" since the Cold War.
He said any extra money should be spent on "new capabilities and not simply plugging gaps".
Labour said real-terms funding for defence had been cut by £9bn since 2010.
The government will decide its spending commitments up to 2021 and potentially beyond in the Comprehensive Spending Review this autumn.
There has not been a full-scale Strategic Defence and Security Review, looking at future defence challenges and capabilities since 2015 and one is expected in 2020.
'Balance of power'
Mr Hunt said it was "not sustainable" to expect the US to spend 4% of its GDP on defence while other Nato allies spent between 1% and 2%.
The UK already spends 2% of its economic output on defence but many European countries do not - although all Nato members have agreed to do this by 2024.
"So for these and other reasons I believe it is time for the next Strategic Defence and Security Review to ask whether, over the coming decade, we should decisively increase the proportion of GDP we devote to defence," he said.
"We simply do not know what the balance of power in the world will be in 25 years' time."
Mr Hunt said the UK currently accounted for almost 20% of total EU defence spending, and British forces contributed a "hugely disproportionate share" of some key capabilities.
But he added that the UK had entered a "multipolar world" without the "assurance provided by unquestioned American dominance".
"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," he said.
On Brexit Mr Hunt said the UK must leave the EU "cleanly and properly", and to fail to do so "would betray the promise of a democracy".
The foreign secretary is among those expected to stand to succeed Theresa May as leader of the Conservative Party when she steps down.
Mrs May has already promised to go once the first stage of Brexit is over. Pressure has grown on the PM to set out a date for her departure following the Conservatives' drubbing at the local elections.
Shadow Defence Secretary Nia Griffith said Mr Hunt had sat in successive cabinets since 2010 which had cut defence spending.
She tweeted: "If he was so bothered, you'd have thought he might have said something a bit sooner?"