Leading members of five major political groups in the European Parliament have addressed a letter to the Council of Europe, urging it to ensure that Turkey releases Selahattin Demirtaş, a former co-chair of the Peoples' Democratic Party (HDP).

The Grand Chamber of the European Court of Human Rights (ECtHR) ruled for the immediate release of Demirtaş in December, stating that Turkey violated ECHR provisions on freedom of expression, reasonable suspicion and the reasonableness of pre-trial detention.

Senior government officials, including President Recep Tayyip Erdoğan, quickly dismissed the judgment, saying that it was not binding.

"Turkey's clear refusal to implement the ECtHR ruling in Selahattin Demirtaş v. Turkey represents a stark violation of the obligations placed on a Member State of the Council of Europe and threatens to undermine the credibility of the Court," said the MEPs in the letter.

"We urge you to use any means at your disposal to ensure Turkey's full compliance with the Grand Chamber's judgment and secure Mr. Demirtaş' immediate release.

"Not only is his present situation a tragic miscarriage of justice on the part of the Turkish authorities that has come at the expense of Mr. Demirtaş and his family; it has also become symbolic for Turkey's backsliding on human rights, democracy and the rule of law."

Demirtaş has been behind bars since November 2016 on "terrorism-related" charges.

Shortly after the ECtHR ruled for his release in November 2018, he was given a prison sentence in another case in what Erdoğan called a "countermove" at the time, hence his imprisonment continued.

After the Grand Chamber decision, a new indictment against him over deadly protests in 2014 was accepted in early January.