Finland begins its final debate in parliament on Tuesday on NATO membership, without waiting for the approval of Turkey and Hungary.

Nor is Helsinki waiting for neighbouring Sweden, which has also been a candidate since last year but is currently facing a Turkish veto.

The government of outgoing Prime Minister Sanna Marin wants to avoid any potential political vacuum, with elections due on April 2.

"We would have hoped to become members of Nato already. Finland and Sweden fulfil all the criteria, as has been mentioned, and we are yet waiting. And of course, this strains the open-door policy of NATO; it's also to do with Nato's credibility," Marin said.

The 200 members of the Finnish parliament, the Eduskunta, are due to begin debating the NATO accession bill on Tuesday, with a vote expected by Wednesday.

The outcome is not in doubt: in a preliminary vote last May there was near-unanimous support, including from parties which had previously been opposed to joining the military alliance. Only a handful of far-left and far-right MPs are expected to vote against it.

The debate coincides with a visit to Finland by NATO's secretary general for meetings with key political leaders.

"The time has come" for Ankara and Budapest to ratify the two Nordic countries' membership bids, Jens Stoltenberg said. "Both Finland and Sweden have accomplished what they promised" to Turkey, he added.

Finland and Sweden decided to turn the page on their decades-long policy of military non-alignment after Russia's full-scale invasion of Ukraine, applying for NATO membership in May 2022.

Twenty-eight of the alliance's 30 members, including the United States, have already ratified the entry of the two Nordic countries.

The Hungarian government, known for its more ambiguous position regarding Moscow, has signalled its approval in principle.

But after a meeting between the Hungarian and Swedish foreign ministers, a government spokesman in Budapest cited "MPs' concerns about the ratification of NATO accession and unfounded lies about the state of democracy in Hungary".

Turkey's opposition to Swedish membership of NATO has centred on Kurdish militants living in the Nordic country. Stockholm's difficulties with Ankara, which culminated in a series of diplomatic incidents in January, have pushed Finland to revise its original desire to join simultaneously with Sweden.

Jens Stoltenberg has acknowledged that the most important thing was not for the countries to join NATO together, but for them to join as soon as possible.

Turkey confirmed on Monday that it could separate Finland's ratification from that of Sweden.

Hungary has so far not mentioned any obstacles to approving the bid, but it remains uncertain.

A majority of Finns (53%) want to join NATO without waiting for Sweden, according to a poll published in early February.

The country, forced into neutrality by Moscow after its war with the Soviet Union during the Second World War, shares the longest European border (1,340 km) with Russia, behind Ukraine.