Suspected diesel laundering plants in Essex and Merseyside, capable of producing 20 million litres of illicit fuel a year, have been dismantled by HM Revenue and Customs (HMRC).
The two sites were discovered to have evaded an estimated £12 million in duty between them.
Last Thursday, Upminster and West Thurrock premises were searched by HMRC and Kent Police officers.
Pat Curtis, National Oils Co-ordinator, HMRC, said:
Investigations into the seizures are continuing.