Turkey's approval last month of plans to develop a shipping canal in Istanbul comparable to the Panama or Suez canals has opened up debate about the 1936 Montreux Convention.

Canal Istanbul is the most ambitious of what President Recep Tayyip Erdogan terms his "crazy projects", which have seen him transform Turkey's infrastructure with new airports, bridges, roads and tunnels during his 18 years in power.

Turkish officials argue that the new canal is vital to take the pressure off Istanbul's Bosphorus Strait, a key route for world trade which saw more than 38,000 vessels pass through last year.

The waterway between Europe and Asia is clogged with maritime traffic and has seen several shipping accidents in recent years.

But opponents say apart from its environmental impact, the new canal venture could undermine the Montreux accord.

The convention guarantees the free passage through the Bosphorus and Dardanelles straits of civilian vessels in times of both peace and war.

It also regulates the use of the strait by military vessels from non-Black Sea states.

The new canal would allow ships to transit between the Mediterranean and the Black Sea without passing through part of the straits that are covered by the treaty.

Ozgur Unluhisarcikli, an analyst with the German Marshall Fund, said whether the new canal would affect the Montreux rules remained "ambigious".

"Montreux is a convention that guarantees Turkey's security. Even if the new canal is not bound by the treaty, Turkey must respect its terms unilaterally," he told AFP.

The proposed 75-billion-lira ($9.8-billion) alternative would run to the west of the Bosphorus along a 45-kilometre (28-mile) route.

Unluhisarcikli said if the new canal is covered by Montreux treaty, Turkey could not demand fees from commercial vessels.

In their letter, 103 retired admirals said it was "worrying" to open the Montreux treaty up to debate, calling it an agreement that "best protects Turkish interests".

"We are of the opinion to refrain from any kind of rhetoric or action that could make the Montreux Convention... a matter of controversy," they said.

- 'Reminiscent of coup times' -

The letter drew a strong riposte from top government officials, while prosecutors in Ankara have also launched an investigation.

"Not only those who signed but also those who encourage them will give an account before justice," Erdogan's top press aide Fahrettin Altun said on Twitter, referring to the probe.

Presidential spokesman Ibrahim Kalin said the declaration was "reminiscent of coup times".

"They should know that our esteemed nation and its representatives will never allow this mentality," he tweeted.

The Turkish military, which has long seen itself as the guarantor of the country's secular constitution, staged three coups between 1960 and 1980.

Erdogan's government also survived an attempted coup in July 2016 which it blamed on followers of US-based Muslim preacher Fethullah Gulen.

Vice-President Fuat Oktay said the putschists were given an "unforgettable lesson on the night of July 15", adding: "And today the most clear response will be given in every platform."

Istanbul mayor Ekrem Imamoglu of the opposition CHP party is one of the strongest opponents of the canal project on financial and environmental grounds.

In November, the interior ministry launched a probe into the mayor over his opposition to the canal.

The investigation by the ministry's property inspectorate is focused on posters containing the phrases, "Either Canal or Istanbul" and "Who needs Canal Istanbul?"