Vladimir Putin shakes hands with Finnish president Sauli Niinisto during their joint press conference in Helsinki in 2019. Photo: Getty Images

Following Russia's invasion of Ukraine, Finnish prime minister Sanna Marin acknowledged the debate surrounding its membership of Nato "will change”, having previously stated it is "very unlikely" Finland – which borders Russia – would apply for a Nato membership during her current term of office.

Polling earlier this month showed around 62% of Finns now back such a move, the first time a majority of the population has done so and up from around 28% in February.

But speaking on BBC’s Newsnight on Tuesday, Finnish president Sauli Niinisto warned Putin could unleash a hybrid war – a mixture of conventional warfare and other methods - if Finland were to press ahead with membership.

Last month, Russia threatened "serious military and political repercussions" if the Nordic country were to join the Atlantic alliance.

Niinisto said that while Finland is a “military non-alliance”, it is not neutral due to its “deep enhanced partnership with Nato”.

He warned: “I don't believe Russia will come with tanks.

“I don't believe there will be a war but I believe there would be a hybrid war… there would be more tension going on.”

Elina Valtonen, an MP for Finland’s National Coalition Party, said that a hybrid war on Finland would mean “other means of interfering with our sovereignty or security”.

She explained: “For instance, sending a number refugees across our border, which has been tried by Putin a number of years ago in northern Finland…

“It can be anything else – for instance interfering with electricity systems or cyber warfare.”

Last month, Russian Foreign Ministry spokeswoman Maria Zakharova said Russia had “noted the targeted efforts of Nato and other members of this alliance to involve Finland as well as Sweden [a fellow non-member] in this alliance".

She warned: "We have seen this policy over a number of years... and Finland and Sweden should not base their security on damaging security of other countries.

"Their accession to Nato can have detrimental consequences... and face military and political consequences."

Part of Putin’s basis for war is to halt what he believes is a Nato advancement into eastern Europe, and demands that Ukraine changes its constitution so that it does not join either Nato or the EU.

However, Ukraine was not on any formal path to joining Nato – a 30-member defensive alliance that includes nuclear powers like the US – before Putin invaded.

Ukrainian president Volodymyr Zelenskyy has now said his country "must admit" that it wouldn't join Nato.

Speaking in a meeting with leaders of the Joint Expeditionary Force this week, Zelenskyy said: "It is clear that Ukraine is not a member of Nato. We understand that. We are adequate people.

"For years, we have heard about the supposedly open door, but we have also heard that we should not enter, and this is true and we must admit it.”

Boris Johnson has also said there is “no way Ukraine is going to join Nato anytime soon” as Zelenskyy said peace talks with Russia had taken a “realistic turn”.

Johnson, speaking to broadcasters at the Emirates Palace hotel in Abu Dhabi on Wednesday, said: “I talked to Volodymyr (Zelenskyy) again yesterday and of course I understand what he is saying about Nato and the reality of the position.

“And everybody has always said – and we’ve made it clear to Putin – that there is no way Ukraine is going to join Nato any time soon.”

But Johnson said decisions about the future of the country had to “be for the Ukrainian people” and their “elected leader” to make, as he vowed Britain would continue to “back” Zelenskyy.

It comes as UK foreign secretary Liz Truss urged allies to step up their measures against Moscow, as she called for those in opposition to the war to sanction Russia.

Truss announced more than 370 Russian and Belarusian sanctions on Tuesday, bringing the total number of individuals, entities, and subsidiaries put under strict measures at more than 1,000 since the invasion of Ukraine began.