The CJEU concludes that national legislation which temporarily limits the period for accrual of interest on the undue payment due to the taxpayer infringes EU law. The principle of effectiveness, in conjunction with the principle of sincere cooperation, must be interpreted as precluding a piece of national legislation which, when a request for a refund of an overpayment of tax is submitted more than 30 days after the publication in the Official Journal of the European Union of a ruling of the Court of Justice from which the finding that the tax at issue is contrary to EU law is derived, limits the running of the interest on the overpayment due to the taxable person concerned to the thirtieth day following that publication, or even excludes interest entirely in a situation where that overpayment was incurred by the taxable person after that thirtieth day.
Consequently, it is up to the domestic legislation of each Member State to regulate the conditions for the payment of such interest in cases of refunds of tax collected in breach of European Union law. This regulation must comply with the principles of equivalence and effectiveness, ensuring that it is neither impossible nor excessively difficult to exercise the rights conferred by European Union law.
However, for taxpayers who did not appeal in a timely manner, it is possible to request a refund of overpaid tax using the procedure for requesting refunds of undue tax payments (article 221.1d) General Tax Law) regulated in article 89 of Legislative Royal Decree 2/2004.