According to Wikipedia eight states are obliged to join the eurozone once they fulfil the strict entry criteria, but this does not include Sweden, “which has a de facto opt out”.
The Wikipedia article Eurozone explains this interpretation in the following way:
Sweden gained a de facto opt-out by using a legal loophole. It is required to join the eurozone as soon as it fulfills the convergence criteria, which includes being part of ERM II for two years, while joining ERM II is voluntary. Sweden has so far decided not to join ERM II.
We can all agree that Sweden is factually outside the euro area, but the European Central Bank (ECB) takes a different view on the admissibility.
The European Central Bank’s Convergence Report May 2010 (273 pages) states in the country summary (page 53):
Sweden is a Member State with a derogation and must therefore comply with all adaptation requirements under Article 131 of the Treaty. Furthermore, the ECB notes that, pursuant to the Treaty, Sweden has been under the obligation to adopt national legislation with a view to integration into the Eurosystem since 1 June 1998. As yet no legislative action has been taken by the Swedish authorities to remedy the incompatibilities described in this and previous reports.
When we read the European Commission’s Convergence Report 2010 (Brussels, 12.5.2010 COM(2010) 238 final;30 pages), we notice that legislation in Sweden is not fully compatible with Articles 130 and 131 TFEU.
Sweden does not fulfil the criterion on price stability, nor the exchange rate criterion (pages 28 to 29).
In the light of its assessment on legal compatibility and on the fulfilment of the convergence criteria, the Commission considers that Sweden does not fulfil the conditions for the adoption of the euro.
Rule of law?
The Swedish government position seems to be that possible treaty compliance is subject to the outcome of a referendum in an unforeseeable future.
Normally, if a member state has failed to fulfil an obligation under the treaties, the Commission delivers a reasoned opinion. In case of non-compliance, the Commission or a member state may bring the matter before the Court of Justice of the European Union (Articles 258 and 259 TFEU).
Sweden has been in breach since 1998, but no Court proceedings seem to be imminent.
Are we to conclude that treaty compliance is optional in the European Union, at least de facto?
The rule of law is mentioned as one of the founding values of the European Union (Article 2 TEU), but would it be more to the point to call it a floundering value?