Twelve of the 103 retired Turkish naval admirals who made a statement on the Montreux Convention last year face charges of conspiracy against the constitutional order and up to 12 years in prison, the daily BirGün reports.

The prosecutor in the case alleges that the 12 admirals sought to mobilize senior military leaders and opposition groups when they drafted a statement accusing the government of "Islamizing" the army and opening the Montreux Convention to debate.

The admirals opposed the Istanbul Canal, an artificial waterway project to create an alternative to Istanbul's Bosphorus Strait, championed by Turkish President Recep Tayyip Erdogan, saying it would undermine the 1936 convention establishing Turkish sovereignty over the Bosphorus and Dardanelles straits. According to them, the Montreux Convention ensures peace in the Black Sea and allows Turkey to remain neutral since World War II.

The other 91 signatories had no intention of engaging in criminal activity and did not collude with the 12 admirals, the prosecutor claims, demanding their acquittal.

There is no legitimate reason in the case, one of the 12 retired admirals, Cem Gurdeniz, wrote on Twitter.

Some of the language used in the declaration drew condemnation from government officials, with Fahrettin Altun, the president's communications director, accusing the admirals of preparing a coup d'état.