HELSINKI (AFP) - Finland's Swedish-speaking autonomous Aaland Islands in the Baltic Sea are due to vote on the Lisbon Treaty later this year, with the outcome seen as uncertain and a No vote likely to land Helsinki in a quagmire.

Politicians in the Finnish capital and Åland's local capital of Mariehamn will be closely following Ireland's second referendum on the Lisbon Treaty on Friday, as Åland seeks to use its own vote as a pretext to wrest more power on EU matters from Helsinki.

Although Finland ratified the Lisbon Treaty in September 2008, the Åland Islands, located between mainland Finland and Sweden and home to 27,000 people, have to approve it by a two-thirds majority in their 30-member parliament to endorse it.

A number of MPs have threatened to vote against the treaty unless their demands are met by Helsinki.

Nobody in Finland seems to know what would happen if the Lisbon Treaty was ratified by all 27 EU member states, but not by Åland.

"That would create a complicated situation. We would have to solve it somehow," Sten Palmgren, a senior legal advisor with the Finnish justice ministry, told AFP.

"It would create a legally problematic, even absurd, situation, because the Finnish state is responsible for all EU laws being followed in the country," said Tuomas Ojanen, a professor of constitutional law at the University of Helsinki.

Any decision by Helsinki to ignore an Åland No would most certainly be perceived as undemocratic at home.

In Brussels, the matter does not appear to be causing too much worry.

"It's an internal problem for Finland" to sort out, one EU diplomat said.

"What matters as far as the EU is concerned is whether a member state has ratified or not at the end of the national process. So if there is a problem, it's up to the Finnish government to solve it," he added.

No date has been set for the Åland vote yet, though it is expected to take place in November or December.

Åland, an entirely Swedish-speaking territory, was Swedish until 1809 when it fell into Russian hands. In 1921, it was "given" to Finland by the League of Nations, which also guaranteed its autonomous status.

Now one of Finland's wealthiest regions, Aaland's economy is driven by the shipping and financial services sectors, but its relations have often been conflictual with Helsinki.

The situation has worsened since Finland joined the EU in 1995, when a series of exemptions for Aaland were negotiated in Finland's membership deal, and there are occasional calls for Aaland to withdraw from the EU or even separate from the rest of Finland.

The most common disputes with Brussels concern fishing and hunting quotas.

But Aaland is most bitter over Finland's decision not to support the islands when they wanted an exemption from an EU ban on "snus", a traditional Swedish moist tobacco for which Sweden negotiated an exemption but which the Commission took Finland to court over.

Finnish President Tarja Halonen asked Aaland to ratify the Lisbon Treaty in April 2008, and Aaland's politicians are now using the vote to press Helsinki to bolster Aaland's influence on EU matters.

The primary issue has been a seat for Aaland in the European Parliament, a demand the Finnish government has outright rejected outright.

However, Helsinki has agreed to allow Åland's representatives to attend European court hearings when the islands are concerned.

The islands also want to be heard in decision-making at the council of ministers.

Meanwhile, Professor Ojanen explained that if Åland did not ratify the treaty, some EU laws that fall under Aaland's autonomy would not come into force on the islands and the Commission could take Finland to court.

Economic, environmental and cultural issues are for example matters that fall under the Aaland parliament's authority.

"Legal uncertainty could cause trouble to entrepreneurs and ordinary people for example in social security or company taxation issues," Ojanen said.

But professor Teuvo Pohjolainen from the University of Joensuu disagreed.

"I don't think it would have a big impact" on people or companies, he said.

Polls in Ireland suggest voters there will approve the treaty in Friday's referendum.

An Irish Yes vote would put Finland to the test later this year, while a No vote would give Helsinki more breathing room to resolve its negotiations with Åland.