Eight men, including two from Co Armagh, have been arrested after 55,000 litres of fuel were seized by customs officials.
In a probe into suspected fuel smuggling fraud worth an estimated £3m in tax evasion, the men were arrested in various locations across the UK and Northern Ireland.
HM Revenue and Customs officers searched one business and four domestic addresses in Knowsley, Liverpool, Manchester and St Helens.
Simultaneously officers from the PSNI carried out searches of six domestic addresses in counties Antrim, Armagh and Down.
The two men from Co Armagh who were arrested are aged 37 and 45.
One man aged 56 from Co Down was also arrested. Another man aged 47 from Co Antrim was arrested.
Three men, aged 34, 54 and 72 from Merseyside were also arrested and one man aged 37 from Manchester was arrested.
Sandra Smith, Assistant Director, Fraud Investigation Service, HMRC, said: “Fuel fraud costs millions of pounds in lost duty every year, meaning the public lose out on vital funds for essential services, and it creates an uneven playing field for honest businesses.
“We will continue to work with our partners to tackle this fraud and ask anyone with information about this type of activity to contact the Customs’ Hotline on 0800 59 5000.”
Storage tanks, equipment, 55,000 litres of fuel, a quantity of cash, computers, business records, a lorry and a trailer were seized during the operation.
Seven men have been released on bail and investigations continue.