In March a sonar scan during the search for a person who fell in the water revealed the intriguing image which led many to believe it could be a wartime vessel.

Londonderry was used as a naval base during World War Two and it was thought that Lough Foyle may have had a war grave beneath its waters.

However, during a visit to Derry Clarendon Street Conservation Area on Thursday, the Environment Minister finally ended speculation when he revealed the results of an investigation into the anomaly in the water.

Alex Attwood said: "Unfortunately, I have to again confirm that the thrilling possibility of a significant wreck has not been realised. The dive was difficult, with minimal visibility, much of the search conducted by finger-tip, with very limited visual inspection.

"However, the dive has concluded that there is a seam or ridge of bedrock/rock outcrop on the riverbed, around which there is various debris.

"There is no photograph to confirm this due to poor water quality - but diver and the sonar images lead to the conclusion that there is nothing of significance on the bed of the Lough at this point," he said.

"As I have said before, I share the sense of disappointment that a submarine or other wreck is not present.

"I want to acknowledge and thank the very many people who have helped in this enterprise - it has excited great public interest, some level of expectation and witnessed the individual and common efforts of many.

The minister added: "I also again want to recall how the search of the river was commenced in the first place - the search for a missing person.

"As the story of the 'submarine that never was' closes, I wish to remember the family of the person who was missing."