Officers from North Wales Police and West Midlands Police targeted a suspected organised crime gang in the West Midlands targeted properties in Smethwick, Dudley, Handsworth and Small Heath were raided on Friday.
The raids followed an extensive joint-policing operation between the forces, with the properties including residential and industrial units where fuel could be held.
Three people were arrested while officers seized a heavy goods vehicle suspected to be linked with the thefts.
The group is suspected of being involved with around 40 thefts of more than £1 million of fuel across the country.
North Wales Police said seven of the incidents were reported within North Wales, prompting action from the forces. In each of the offences in its patch, committed at industrial units in Wrexham, Flintshire and Anglesey, approximately £50,000 worth of fuel was stolen.
Other thefts have also taken place in Hampshire, Surrey, Kent, West Mercia and Scotland.
Thieves used large vehicle batteries to power pump systems which moved the fuel from storage tanks to large intermediate bulk containers (IBCs). Using these devices enabled the suspects to syphon around 10,000 litres of fuel per hour at each offence location.
DCI Alun Oldfield, from North West Police, said: "(The) arrests are the result of an intensive year-long investigation, which sought to identify those responsible for a spate of large-scale fuel thefts.
“Led by North Wales Police officers, this cross-force operation culminated in the execution of targeted warrants across the Birmingham area.
“I would like to thank West Midlands Police officers and the Customs and Excise staff who assisted in this operation.
“Victims incurred significant financial losses as a result of these crimes and today’s arrests will disrupt the co-ordinated planning of future fuel thefts across the UK.”
The arrested suspects remain in custody and the investigation remains ongoing.