THE wreck of a German U-boat that sank almost 100 years ago has been discovered by engineers laying subsea power cables.

Remarkable sonar images show the missing World War One submarine is largely intact and lying off the Galloway coast.

The BBC said experts believe the vessel could be the UB-85, a sub that sunk in 1918, according to official records.

Dr Innes McCartney, a historian and nautical archaeologist helped to identify the wreckage.

Dr McCartney said there were at least 12 British and German submarines known to have sunk in the Irish Sea.

He said: "The features of this particular wreck, which is largely intact, confirm it as a UBIII-Class submarine, of which we know of two which were lost in the area - the more famous UB-85 and its sister boat UB-82.

"While I can conclude that this wreck is likely to be one or the other, they would be practically impossible to tell apart, aside from the numbers painted on them in service, now obviously long gone.

"Unless a diver can find a shipyard stamp, we cannot say definitively, but yes, we're certainly closer to solving the so-called mystery of UB-85 and the reason behind its sinking - whether common mechanical failure or something that is less easily explained."

The historic discovery was made by engineers involved in the £1bn Western Link project to lay a subsea power line between Ayrshire and the Wirral.

The 385km (239miles) long cable will carry renewable energy produced in Scotland to England and Wales.

The engineers found the wreckage 120m north-west of the centre of the planned route, off the Stranraer coast. It is about 45m long, with debris spilling from the stern.

Scottish Power Transmission and the National Grid are working together on the Western Link project to lay the undersea cable.

Peter Roper, of Scottish Power, said: "The images we get back from the subsea scans are incredibly detailed, but we obviously need to be aware of what lies beneath before we can start laying a power cable.

"In all the years I have been building power lines, I can say that this is the most extraordinary discovery."