Turkey has issued new regulations governing the Turkish straits that appear to change a 46-year-old international agreement by asserting that the Istanbul port authority has the right to temporarily block shipping in the strategic waterway.

The straits, the only waterway between the Black Sea and the Mediterranean, are heavily used by the Soviet Union and other important shipping nations including the United States, Britain and France.

The 1936 Montreux Convention states that "in time of peace, merchant vessels shall enjoy complete freedom of transit and navigation in the straits, by day and by night, under any flag and with any kind of cargo, without any formalities." Under the new regulations, announced here Wednesday, "transit through the straits of Istanbul can be suspended temporarily by the port authority if necessary."

The Montreux Convention applies to the Turkish Straits as a whole, including the 36-nautical-mile Dardanelles between the Aegean Sea and the Sea of Marmara, and the 17-mile Bosporus linking the Sea of Marmara and the Black Sea. The new regulations apply to the Bosporus, or straits of Istanbul.

The Turkish government did not issue an explanatory announcement. However, it is believed that the new regulations, which also change transit lanes and bring a host of other new measures, were intended to protect Istanbul, where 5 million people live. Recently there have been numerous accidents in the straits involving foreign ships, highlighting the danger of fire in the city.

Under the Montreux Convention, "pilotage and towage remain optional" for commercial vessels. The new regulations make piloting and towing charges "compulsory" for specified transit and anchorage routes.

The Montreux Convention was signed by Bulgaria, France, Great Britain, Greece, Japan, Romania, Turkey, the Soviet Union and Yugoslavia and has never been amended.