Donald Trump's suggestion that he would encourage Russia to invade Nato allies who do not meet defence spending commitments was "not a sensible approach", Lord David Cameron has said.
Speaking to reporters as he visited East Kilbride, Scotland, the Foreign Secretary said: "I am a very strong supporter of Nato.
"It is what helps to keep us safe and that is so essential in this world where we have seen Putin's terrible illegal invasion of Ukraine.
"And actually Nato this year has got stronger, with Sweden and Finland joining.
"Of course we want all countries like us to spend 2% (of GDP), but I think what was said was not a sensible approach."
While some of the White House candidate’s Republican allies rallied round his inflammatory remarks, President Joe Biden’s administration called them "appalling and unhinged".
“No, I would not protect you,” Mr Trump said at a weekend campaign rally as he relayed conversations he had with a Nato leader in Europe when he was president, demanding the country spend at least 2% of its GDP on defence.
European Union foreign policy chief Josep Borrell responded: “Nato cannot be an a la carte military alliance... depending on the humour of the president of the US.”
Britain’s security minister Tom Tugendhat stressed that the US people had made clear their commitment to the alliance “over many, many years” through the deployment of millions of troops.
He said on LBC: “We need to make absolutely certain that all Nato members are [meeting the target] because the reality is, I'm afraid, that Putin's war against Ukraine could spread if we are not successful and we need to make sure that we're all defended.”
Nato Secretary-General Jens Stoltenberg said on Sunday that the 31 allies remained committed to defending each other.
"Nato remains ready and able to defend all allies. Any attack on Nato will be met with a united and forceful response,” he said. “Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk.”
The Kremlin declined to comment, but other Republicans blasted Mr Trump.
"This is why I've been saying for a long time that he's unfit to be president of the United States," former presidential candidate Chris Christie said on NBC.
Speaking at the rally in South Carolina on Saturday, Mr Trump recounted: “One of the presidents of a big country stood up and said, Well, sir, if we don't pay and were attacked by Russia, will you protect us?
“I said.. No I would not protect you. In fact, I would encourage them to do whatever the hell they want. You gotta pay. You gotta pay your bills.”
The attack appeared to reflect Mr Trump’s well-worn frustration with other Nato countries for not spending at least 2% on defence. The alliance estimates that only 11 of the 31 member states are spending at the target level.
Under Nato’s mutual defence clause, called Article 5, all allies commit to help defend any member who comes under attack. Article 5 has only been invoked once, to aid the United States after 9/11.
Senator Marco Rubio, a Trump ally, said there was nothing new in Mr Trump’s frustration.
"Virtually every American president at some point, in some way, has complained about other countries in Nato not doing enough. Trump's just the first one to express it in these terms. But I have zero concern, because he's been president before," the Florida senator told CNN.
But White House spokesman Andrew Bates commented: “Encouraging invasions of our closest allies by murderous regimes endangers American national security, global stability and our economy at home.
"Rather than calling for wars and promoting deranged chaos, President Biden will continue to bolster American leadership and stand up for our national security interests - not against them."