More than 1,000 litres of red diesel have been found by Northants Police and customs officials.
Police and HM Revenue & Customs officers searched a property in Southfields, Northampton, and discovered 1,400 litres of red diesel, stored in metal drums and plastic containers.
Two domestic cars at the same property were found to be fuelled by red diesel.
Officers were acting on a tip-off they had received about fuel thefts.
Customs officials are now leading the investigation.