Security at a Devon naval base and aboard a warship is being investigated after an undercover reporter gained access using a lost cleaner's pass.
The reporter said he was only stopped once on board the ship
The Sun reporter spent one hour aboard HMS Ocean at the Devonport naval base in Plymouth on Tuesday and took video footage of the deck.
He said guards waved him into the base without closely inspecting his pass.
A Ministry of Defence spokeswoman said a full investigation by both the ship and the naval base had begun.
She said: "We take all security breaches very seriously and keep security procedures under constant review."
The journalist said he was only stopped once and when he explained he was a cleaner, was sent on his way without further questioning.
HMS Ocean can carry a sea crew of 255, an air crew of 206 and up to 480 Royal Marine commandos.