Alex Attwood has banned diving at the site of the discovery near the Foyle Bridge. He said it was important that the wreck was not disturbed until it was positively identified.  "The wreck is in quite low water, it is quite accessible," Mr Attwood told the BBC.

BBC Northern Ireland Environment Correspondent Mike McKimm said that was unlikely to be a full-sized submarine.

"A German U-boat, for example, would sit almost 10m high and would have been visible, even at high tide in the Foyle, which has an average depth of just over 5m."

He said there was speculation that the vessel could be a mini or midget submarine from the war.

"These would be just a couple of metres high. In 1944 the German navy operated a very successful two-man mini-sub, the Seehund.

The vessel could also be a British two-man mini submarine - known as an X craft, a small boat or even a private yacht - one is known to have sunk in the area many years ago.

"It's all speculation until divers can reach the wreck," Mike McKimm said.

"Diving is not expected to start for a week or so. Underwater visibility in the Foyle is very poor at the best of times making identification difficult.

I have asked that urgent work is undertaken to determine how and when the submarine or boat came to rest in this part of the lough. It is unusual that there appears to be no record of the circumstances behind this situation.

Minister Alex Attwood