Border Force investigators made an arrest in Harrow in a series of dawn raids this morning.
As part of an investigation into an alleged criminal money laundering network four people were arrested.
Arrests of three men aged 63, 38 and 45, and a 33-year-old woman, were made in Weybridge, Harrow, Teddington and Epsom.
The investigation began after Border Force officers at St Pancras International rail station seized £300,000 in cash from a passenger in September 2011.
It is suspected that a company may have been used to launder significant amounts of criminal cash and transport it illegally across international borders.
Those arrested this morning are now being questioned.
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 Her Majesty's 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.”
Anyone with information about activity they suspect may be linked to smuggling should call 0800 59 5000.