Navy chiefs are said to be "furious" after allegations that £250,000 of fuel had been stolen from a warship, say reports.
The diesel was meant for HMS Bulwark, but it is now feared the thefts were going on for weeks.
An investigation is under way after a tanker was reportedly stopped trying to leave HMNB Devonport in Plymouth, The Sun reports.
A source told the outlet a sentry became suspicious and carried out the search - potentially uncovering the massive theft.
The insider stated: “Naturally, the Navy is furious about it — even though none of their personnel were involved.”
It is claimed the fuel was intended to fill up generators to power the Navy vessel.
MP Luke Pollard, who represents Plymouth Sutton and Devonport told The Sun: "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 security at one of our most secure naval bases.”
And Rear Admiral Dr Chris Parry described it as "incredibly careless".
According to reports much of the fuel has already been sold on the black market, at a time when petrol prices are soaring.
The Ministry of Defence said in a statement: "‘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.”
Devon and Cornwall police have been contacted for comment.
