Calcutta, Sept 24 - Frogmen, apparently members of the East Bengal resistance movement, have severely damaged a 16,000-ton British cargo ship, the Teviotbank, which was loading jute off the East Pakistan port of Chalna last Sunday night.

According to British seamen who have just arrived in Calcutta the frogmen attacked the Teviotbank on Sunday night with limpet mines. The master, Captain B. Patterson, is still desperately attempting to keep her afloat.

As a result of the attack, another British ship, the 10,000-ton Chakdina, weighed anchor suddenly on Monday and attempted to flee from Chalna port. She was attacked by small arms fire but apparently there were no injuries or damage.

A spokesman for the provisional government of Bangla Desh said today that the Teviotbank was not attacked because she was British. The action was part of the guerrillas' general objective of breaking Pakistan's economy, by disrupting communications and shipping. The spokesman warned shipping lines that any vessel entering the ports of Chittagong and Chalna in East Pakistan would be liable to sabotage, although ships carrying relief food and supplies would not be attacked.

It is understood that the representatives of several main lines have advised their principals that it might be wise to suspend their operations in East Pakistan ports.

According to officers and seamen who sail regularly between Calcutta and East Pakistan the guerrillas have launched several successful attacks against commercial ships in East Pakistan ports during the past five weeks. Apparently limpet mines have been used by professional frogmen.

Members of the crews of several ships which have recently sailed from East Pakistan report that besides the Teviotbank several vessels, including two ocean-going steamers, a Pakistan coastal tanker and several barges, are lying damaged in Chalna port. Crew members of the Chakdina also said that East Bengali frogmen sank and damaged two Pakistan cargo vessels in Chittagong port five weeks ago. The 10,000-ton Ohrmazo sank at the bows after limpet mines holed her beneath the waterline on August 14.

One officer said: "Whenever we pull into Pakistan ports now Pakistani soldiers come on board to guard the ship against saboteurs."

Many informants say that although ports are functioning. conditions on the wharves are far from normal. In Chittagong the ports are suffering from a labour and wagon shortage. A curfew is imposed at night and offloading is painfully slow.

About three weeks ago mines were removed from a Greek ship in Chalna and the ports are now patrolled at night by Pakistan gunboats with searchlights.

Although they have suffered severe casualties the East Bengal guerrillas have also been far more successful in launching land operations against the West Pakistanis than observers at first expected.

Michael Baily writes: A Bank Line spokesman in London confirmed that the Teviotbank had been holed in two places and temporary repairs were being carried out with the help of the Pakistan Navy. Representations had been made through the Government to East Pakistan.