More than 5,000 litres of diesel has been stolen from Coventry Hill Service Station at Hullbridge, near Rayleigh, Essex.
Staff discovered the theft on Sunday morning and found that 5,620 litres had been taken.
Police said that two men parked a Luton van on the forecourt just after midnight on Saturday, and they are believed to have pumped the fuel into tanks in the vehicle over a period of about two hours.
PC Ahren Perry said: “Lower Road is a busy road and we know that many vehicles would have driven past during the two hours the thieves were on the site. We also know that a number of pedestrians walked past. We want to hear from anyone who has information about the van and the two men.
“We also hope that drivers might remember seeing the van after it was loaded with fuel because 5,620 litres of diesel fuel would be a heavy load for a Luton van and it would have been very low on its suspension.”
Anyone with information should contact PC Perry at Rayleigh Police Station on 101 or they can call Crimestoppers on 0800 555111.