More than £250,000 worth of diesel has reportedly been stolen at the Royal Navy's HMNB Devonport base in Plymouth.
The theft is understood to have involved fuel that was being used for generators on the naval base and is thought to have taken place over a number of weeks.
The figure has not been officially confirmed.
Babcock International, who work with the Ministry of Defence at Devonport declined to comment but did confirm that there was a police investigation taking place.
MP for Plymouth Sutton and Devonport said: “The theft of fuel from a Royal Navy warship is not only criminal, it takes scarce resources away from our military in a time of national crisis.
"With the armed forces facing more and more cuts, having such a huge amount of fuel stolen is not only embarrassing, but it also raises serious questions about the security at one of our most secure naval bases.”
An MoD spokesperson reportedly told The Sun on Wednesday evening: "The MOD is aware of an incident involving the alleged theft of fuel from a contractor within HMNB Devonport.
"There was no disruption to Defence operations and the MOD has no further comment.”