A German U-boat sunk off the Scottish coast more than 90 years ago has finally been discovered by two divers after a five-year search.

Jim MacLeod and Martin Sinclair found the wreckage of the U12, the first ever submarine to have an aeroplane carried on its deck, 25 miles from the Berwickshire port of Eyemouth at the weekend.

The exact location of the 60-metre boat had become a mystery to the two divers after a number of searches of the seabed where it was recorded to have been lying proved fruitless.

The pair worked with a researcher and shipwreck enthusiast Kevin Heath, of Orkney, who tracked down the logbooks of British destroyers HMS Ariel, Acheron and Attack, all of which were involved in the sinking of the U12.

The precise location of the vessel was then pinpointed, 15 miles from where it was originally thought to be.

Mr MacLeod, 45, a computer systems analyst from Bo'ness, and Mr Sinclair, 47, a mechanical engineer from Falkirk, then enlisted the help of specialist Eyemouth firm Marine Quest Dive Charters to visit the location where they found the submarine lying 150 feet down on the seabed.

It was the first time the wreck had been visited since it was sunk in 1915.

The discovery has been reported to the German authorities as 19 sailors died on the vessel and the submarine has now been declared an official war grave. The submarine will remain untouched and relatives of its dead crew will be informed of its find.

Mr MacLeod said: "It has taken a lot of effort and perseverance but it has finally paid off.

"Although the name of the boat has been rusted off, we were able to identify it as it was quite unique in its day with its stern torpedo tubes, conning tower and submarine potholes which were a bit pointless, frankly.

"It was the first ever submarine to launch a plane at sea. It would go so far on the surface and then, when it came into the range of the enemy, would dive down allowing the plane to take off, but it was an experiment which never took off.

"It is a war grave so there was no disturbance of the site or removal of artefacts. We are delighted with the find."

Mr Heath, 44, from Stromness, Orkney, a scallop diver by profession, put the true location of the submarine on the map after tracking down the relevant naval documents.

Mr Heath said: "This is a very significant find as there are not that many unaccounted for.

"The U12 had been targeting cargo ships off the east coast of Britain and had been as far north as Peterhead. It had sunk a boat the previous night but on March 10, 1915, it was hunted down by the three British destroyers HMS Ariel, Acheron and Attack.

"It attempted to dive under the surface but it was rammed by HMS Ariel.

"The submarine then surfaced and was being shelled by Acheron and Attack when it was realised they were surrendering. However, the conning tower hatch jammed and 19 sailors drowned and there were 10 survivors who got out the other hatches.

"They surrendered but it led to a diplomatic incident. They were treated as pirates by the British for the way they were targeting cargo ships and put into solitary confinement.

"This angered the Germans who did the same with British prisoners of war and the situation had to be dealt with by the Swiss."

The wreck itself has great historical importance as it was one of few which carried an aeroplane on its deck, a feature which turned out to be a limited experiment.