Dozens of retired Turkish admirals warned in an open letter that their country’s grand plan to dig a new canal alongside the Bosphorus could threaten the free passage of ships between the Black Sea and the Mediterranean.

Turkish authorities, however, dismissed the letter and detained some of its signatories, claiming that their actions were politically motivated and could be a precursor to a military coup.

The $10bn Istanbul Canal plan is the most ambitious in a series of infrastructure projects that have transformed the Turkish Straits since President Recep Tayyip Erdogan’s AK party won power in 2003.

The planned 45 km canal would bypass the existing Bosphorus waterway, which is often congested, and poses what some believe is an environmental threat for Istanbul, the city of 15m inhabitants that it crosses.

Nearly 40,000 ships transit the Bosphorus each year. Given its commercial and military importance, free right of passage through the waterway is enshrined in the 1936 international treaty of Montreux.

In their open letter, 103 retired admirals warned that the Istanbul Canal plan could undermine that treaty and, indirectly, the Turkish security interests it protects.

Turkish foreign minister Mevlut Cavusoglu, however, dismissed that claim.

"Canal Istanbul has no effect on Montreux and Montreux has no effect on Canal Istanbul,” Cavusoglu said in a report by the country’s state-run Anadolu Agency (AA) news provider.

The Canal Istanbul website already addresses concerns about the compatibility of the project with the Montreux treaty.

"Since the construction of the Istanbul Canal project is planned not to completely eliminate the passage through the Turkish Straits but only bypassing the Bosphorus part, the rules stipulated by the Montreux Convention will continue to apply," the website said.

The Turkish Straits consist of the Bosphorus, as well as the Sea of Marmara and the Hellespont strait further south.

Istanbul Canal snapshot

The proposed Istanbul Canal would cut through Istanbul, drawing some shipping traffic from the Bosphorus.

Depth: 25 metres.

Width: Between 250 metres and 1 km.

Length: 45 km.

Meanwhile, state prosecutors started clamping down on the letters' signatories. Police have detained 10 of them as part of an investigation into whether their action infringes on state security and the constitutional order, AA reported.

The crackdown reflects government concerns that the admirals’ letter might be seen as an indirect call to army officers to intervene in politics.

Thousands of confirmed or suspected opponents of president Erdogan’s regime have been arrested since a failed military coup against him five years ago.

Plans for the Istanbul Canal first emerged in 2011, with work at one time mooted for 2013. The new waterway would run from the Durusu region on Istanbul’s Black Sea coast to Kucukcekmece Lake on the Sea of Marmara.