It also said parliament was wrong in stripping Democrats (SDS) leader Janez Janša of his seat in October last year.

In a review started at Janša's request, the Constitutional Court found that the provisions of the act dealing with the termination of a term of an MP who has been convicted is unconstitutional due to a lack of legal recourse available to MPs.

It tasked the National Assembly with amending the act within a year to include legal safeguards for MPs who have their terms stripped by parliament.

Janša petitioned the court after his status was revoked by parliament in October in an interpretation of the provision which says that MPs who have been convicted to a jail term of more than six months are banned from serving their term.

He argued that the provision applies to MPs who have been convicted mid-term while he was elected to office by voters who were already aware of his conviction in casting their ballots.

The court sided with his argument in finding that the decree stripping him of the term was an unacceptable breach of his right to serve the term in office which he won in a democratic election.

The Constitutional Court said that article 9 of the act "cannot be applied to cases when the cause for termination had existed even before the candidature for office was confirmed".

This does not mean that parliament cannot set limitations on who can be elected to office in a separate law. "As the legislator had failed to settle the issue of non-electability, it cannot extend the provisions related to conditions for ending terms mid-stream to cases where the cause exists during the filing of candidacies."

Already in April, Janša had his conviction in the Patria bribery case repealed by the Constitutional Court, which ordered a retrial in the case. In November 2014, the court temporarily restored his status as MP pending a final ruling.

In ruling today that the act on MPs had a major shortfall in the part dealing with the ending of terms for serving MPs, the court said that legal recourse available to banned MPs is currently lengthy and complex and thus not an effective remedy.

It said that fixes to the act should provide for quick deliberations on disputes regarding the status of an MP and should ensure that such cases are dealt with directly by the Supreme Court or even the Constitutional Court.

Responding to the ruling, MP of the coalition Modern Centre Party (SMC) Lilijana Kozlovič said that parliament faced "an extremely difficult task" in deciding on whether Janša's term should end due to his conviction.

Kozlovič believes the complexity of the issue is reflected in the fact that the Constitutional Court needed more than six months to rule. From its decision, it is clear that there is a legal vacuum in this field which needs to be addressed, she added.

The opposition Alliance of Alenka Bratušek (ZaAB) meanwhile said it was not surprised by the ruling, highlighting that it had sought to introduce legislation which clearly defined conditions for standing for office.

ZaAB said that legislative fixes must now tackle the issue raised by the court in providing for legal recourse, while also determining conditions for standing for office.

A number of other parties said they would comment on the ruling after they study it in more detail.

Janša had previously responded to the ruling in a tweet which apparently alluded to the Constitutional Court ruling before it had become public.

He included the Constitutional Court issuing a slap to the ruling coalition as one of the possible answers in wondering why a story was run today by the daily Dnevnik about the Austrian Supreme Court upholding the bribery conviction of an Austrian middleman in the Patria deal.