Four people have been arrested in raids following an investigation into a suspected multimillion-pound money laundering network, the Border Force said.
Members are believed to have used a company to transport criminal proceeds illegally across international borders.
Three men aged 63, 38 and 45 and a 33-year-old woman are being questioned on suspicion of money laundering after officers swooped on properties in and around London.
A number of documents, mobile phones and computers were picked up as part of the investigation which began when Border Force officers seized around £300,000 in cash from a passenger at St Pancras International rail station last September.
Border Force senior investigating officer Peter Avery said: "This has been a complex financial investigation into allegations that millions of pounds of criminal money have been laundered.
"It has involved close co-operation between Border Force, various police forces and HM Revenue and Customs.
"In addition to making these arrests, we have seized documents, mobile phones and computers, and they will now be analysed as part of our investigation."
The 63-year-old was arrested at a home in Weybridge, Surrey; the 38-year-old was held at a home in Harrow, north London; and the 45-year-old was arrested at a home in Teddington, south west London.
The woman was held by police at a business address in Hampton Hill, also in south west London.
Officers have also carried out searches at business addresses in Hampton Hill and London Docklands, south east London.