A UK navy bomb disposal squad was called in after a Norwegian tanker's anchor dragged up a test torpedo.
Pictures showed the anchor of the 4,300-dwt Brovigtank ship Skaw Provider (built 2005) had speared the munition off Portland.
The Daily Mail reported that crew had spotted the corroded torpedo after raising the chain.
Navy divers went into action to protect the tanker and its 1,000 tonnes of fuel.
Oil was pumped to rear tanks as a precaution.
The device was destroyed after a five-hour operation.
The navy said the torpoedo was British and probably came from a test range at Portland which was in use until the 1980s.
It had no warhead, but still contained highly flammable propellants.
