The shelter was found on Scottish Water land that was once part of the site of RAF Montrose, the first operational military airfield in Great Britain.
Several air raid shelters and gun emplacements can be found around the old airfield, a major training centre for pilots during both world wars, but Scottish Water said the find had been an “unexpected discovery.”
Scottish Water Horizons Project Manager, Mari Davies, said: “We knew that in the past the area could have been home to munitions and radioactive material, all remnants of the Second World War.
“Thankfully nothing hazardous was uncovered which meant we were able to carry on with the work. However once we’d cleared the land, we made the unexpected discovery of a WW2 air raid shelter.
“Although an interesting find, it did mean we have to alter our plans slightly to fit round it.”
Scottish Water is currently installing solar panels to power its water treatment works.
RAF Montrose was created in 1912 as one of 12 “Air Stations” to be operated by the Royal Flying Corps.
Its North East location was designed to protect Royal Navy bases at Rosyth, Cromarty and Scapa Flow.
Originally located at a farm just south of the town, it was moved to its current location in 1913 given the flat ground, the well drained sandy soil by the beach and its easy access to the rail line.
The airfield is said to be haunted by the ghost of Lt Desmond Arthur, who died in a crash on 27th May 1913, with many sightings of apparitions reported.
The official report into his death cited the cause of crash as a bad aircraft repair but this was later overturned to state he had died as a result of stunt flying.
He was later exonerated, but some believe the spirit of the pilot did not rest as a result of the false claims against him.
Ms Davies said: “There have been many sightings of him over the years, but luckily none of our contractors had any paranormal experiences.”
No 2 Squadron was initially based at RAF Montrose until the outbreak of WWI with US soldiers arriving there in 1918 to train for the Western Front.
During World War 2, airmen from Poland, Czechoslovakia, Russia and Turkey were among those sent to the airfield to train.
Spitfires and Hurricanes which formed part of the air defence of Edinburgh were also stationed there.
In October 1940, three German Junkers Ju 88 aircraft dropped 24 bombs on the station killing five, injuring 18 and destroying two hangars and the officers mess.