The operation resulted in 110 arrests – mostly in Spain and the UK, the BBC reports.

Police targeted the masterminds and facilitators of the “boiler room” fraud – so-called because of the cramped conditions they work from.

There are 850 confirmed victims of the gangs in the UK, but the real figure is likely to be in the “multi-thousands”.

Detectives said the aim of the two-year investigation, codenamed Operation Rico, is to “decimate” boiler room fraud in Europe.

They believe it is the biggest ever operation into the crime.

Operation Rico, which culminated in a series of raids this week, was led by City of London Police.

Commander Steve Head, who is the national economic crime co-ordinator, said: “It is our most important investigation ever, targeting people we believe are at the top of an organised crime network that has been facilitating boiler rooms across Europe and which is suspected of being responsible for millions of pounds of investment fraud.”

Among those under investigation were 10 “tier one criminals” with alleged links to organised crime and drugs, detectives said. Nine of them are British, one is South African.

The operation was supported by Spanish police and involved agencies in the UK and overseas, including the US Secret Service.