In one month, the numbers in favour of membership jumped nine percentage points to an all-time high of 51 percent.

Those opposed fell to 27 percent, a ten-point drop, the survey conducted by the Demoskop Institute and published by Aftonbladet newspaper showed.

The number of undecided remained mostly stable at 22 percent.

Another poll published Friday by broadcaster SVT showed a strong increase in support for membership, but fell just short of a majority at 49 percent. Some 27 percent were opposed.

The war in Ukraine has also tipped the scales in Finland, with a majority for the first time seen in a poll this week.

For the time being, Stockholm and Helsinki have ruled out applying to join the NATO military alliance, but support for the move is at historically high levels.

Finland's president Sauli Niinisto, who is meeting US President Joe Biden in Washington on Friday, urged Finns to "keep a cool head" when addressing the issue.

Sweden and Finland are officially non-aligned, although both have been NATO partners since the mid-1990s and turned the page on their neutrality at the end of the Cold War.

In Sweden, opinion was overwhelmingly against NATO membership until Russia's annexation of Crimea in 2014.

Experts expect Finland to act in concert with neighbouring Sweden on whether to join NATO.

The eastward expansion of the alliance is the prime security grievance of the Kremlin and last week Russia's foreign ministry warned that if the Nordic countries joined NATO it would "have serious military and political repercussions".