Boris Johnson has urged all members of NATO to meet the alliance's guideline defence spend of 2% GDP.
The Prime Minister said countries cannot be "complacent" about the need to invest, in order for the military group can continue for the next 70 years.
Leaders have been meeting at a 70th-anniversary NATO gathering near Watford.
His comments came as Donald Trump dramatically cancelled his scheduled press conference, saying he had already done "so many" in the past two days.
The US president had earlier accused Justin Trudeau of being "two-faced" after video footage surfaced, which, it has been suggested, appears to show Mr Trudeau talking about his American counterpart
Speaking of Mr Trudeau, President Trump said: ""I find him to be a very nice guy but you know the truth is that I called him out the fact that he's not paying 2% and I guess he's not very happy about it."
The latest estimates from NATO suggest Canada pays 1.31% of its GDP towards defence - below the alliance's guideline spend of 2%.
According to statistics, the United States spends 3.42% of its GDP on defence, making it the highest contributor in NATO, in terms of a percentage of GDP.
Mr Johnson said: "I certainly think that the United States is the guarantor of, has been a massive contributor to NATO, has been for 70 years a pillar of stability for our collective security," he told a press conference at the Watford summit.
He later continued: "The UK has long argued that you can't be complacent about that, you can't remotely take that for granted.
"You have got to ensure that we continue to spend on our collective defence and that's why we have made the case for 2% of GDP is the minimum NATO spend for every member."