Experts have detonated a 1200kg Nazi bomb after it was discovered by an English teenager who was scuba diving off the Devon coast.

An underwater photograph of James Cunningham, 19, showed the teenager floating above the World War Two bomb, which sits on the seabed encrusted in moss and barnacles.

Cunningham stumbled across the large Luftwaffe bomb underneath a popular pier at Teignmouth beach in Devon, while diving with his uncle on Tuesday morning (AEDT).

Today a Royal Navy bomb disposal crew evacuated the area and took the bomb further out to sea so it could be detonated safely.

Cunningham, a regular diver in the area, said he saw the strange object from some distance and went to explore.

"At first I thought it possibly looked like some sort of container that was covered in rust," he told local media.

HM Coastguard, @DC_Police and the @royalNavy Explosives Ordnance Disposal (EOD) team are currently on scene at #Teignmouth dealing with an unexploded WW1 device found 1km off shore. Public and vessels asked to avoid area as a safety precaution #keepsafe

— Maritime&Coastguard (@MCA_media) July 10, 2018

"Only by looking a bit closer we then discovered that it was in fact a bomb. We swam away post-haste and went back to shore to contact the police."

"I had taken a bearing so I was able to go back out with the Navy this morning to show them exactly where it was."

Until it was detonated, police enforced a 1.5km exclusion area around the bomb and trains were also halted.