Canada’s prime minister, Mark Carney, has said the inadvertent leak of classified military plans by senior US officials means that allied nations must increasingly “look out for ourselves” as trust frays with a once-close ally.

Speaking a day after it was revealed that a journalist was accidentally included in a group chat discussing airstrikes against Yemeni rebels, Carney said the intelligence blunder was a “serious, serious issue and all lessons must be taken”. He said it would be critical to see “how people react to those mistakes and how they tighten them up”.

Canada is one of the members of the Five Eyes intelligence sharing network, alongside Australia, New Zealand and the UK and the leak of classified information is likely to put further strain on the group as it weighs how seriously the current American administration takes the handling of top secret information.

The revelations came as Canada grapples with a rapidly deteriorating relationship with the United States, its largest trading partner and closest military ally.

“My responsibility is to plan for the worst, is to think about the most difficult evolution of the new threat environment, what it means for Canada and how do we best protect Canada,” Carney said during a campaign stop on Tuesday. The prime minister called a snap election on Sunday.

“Part of that response is to be more and more Canadian in our defence capabilities, more and more Canadian in our decisions … We have to look out for ourselves.”

Asked about the incident on Tuesday, the UK’s armed forces minister, Luke Pollard, told the Commons Defence Committee that no British service personnel had been put at risk as a result.

He added: “All UK service personnel are covered by our normal approach to operational security, and the committee will understand that I won’t go into the details of how we keep our involvement in any support for military operations in the Red Sea or anywhere else [secure].

“But we’ve got high confidence that the measures that we have got with our allies, including the United States, remain intact.”

A spokesperson for Prime Minister Keir Starmer spoke at length at a briefing about the contribution the UK makes to joint military operations with the Americans. However, the spokesperson refused to directly criticise the two figures who were most critical of Europe’s record on defence, JD Vance, the vice-president, and Pete Hegseth, the defence secretary. The spokesperson also insisted that the UK remains happy to share intelligence with the US despite the leak.

The government of New Zealand declined to comment on the matter. When asked by the Guardian if the security breach had raised concerns about the sharing of sensitive intel with Trump’s administration, the offices for New Zealand’s prime minister, Christopher Luxon, and the minister for defence, Judith Collins, said the situation was “a matter for the US administration”.

Behind closed doors, senior government officials would likely be discussing the risks of sharing intelligence with the US, amid what could be viewed as a lowering of protocol standards, but the breach would not be a dealbreaker, said Andrew Little, whose ministerial roles covered security, intelligence and defence under New Zealand’s last Labour government.

“Our relationship transcends individual administrations and individual political leaders. There will be things that – like everybody – members of this government, will be looking askance at. But I think it’s about managing the relationship in the long run,” Little said.

So far, New Zealand has been managing its US relationship responsibly, Little said, but it was now “a relationship that requires constant vigilance”.

Robert Patman, a professor at the University of Otago in Dunedin who specialises in international relations, called the security breach “extraordinary” and “cavalier”. “It does confirm what many of us felt, that Mr Trump has picked people according to loyalty, rather than competence, and this was almost a perfect storm waiting to happen,” Patman said.

But the wider issue for New Zealand and other Five Eyes countries was knowing how to respond to the Trump administration’s “radical departure” from the rules-based order, which included making territorial claims against liberal democracies and siding with Russian president Vladimir Putin over negotiations in Ukraine.

“We should be friendly towards the Trump administration where our interests converge, but this administration is doing things which are fundamentally a challenge to [New Zealand’s] national interests.”

In Australia, the department of foreign affairs and trade said: “This incident is a matter for the United States. Australia and the United States engage regularly on implementation of mutually recognised standards for the protection of classified material.”

Ben Doherty contributed additional reporting