Briton Richard Pope faces up to 20 years in a US jail after admitting his role in a multi-million pound scam.
He and his gang fleeced unsuspecting UK pensioners by stealing the identities of dormant US companies and selling non-existent shares, a court heard.
Pope, 53, funded a lifestyle of fast cars and expensive yachts at the expense of his victims, some of whom were left destitute, police said.
So-called boiler room scams are thought to cost the UK about £200m a year.
Pope, originally from St Albans, Hertfordshire, pleaded guilty on Thursday at Middle District of Florida District Court, to committing mail and wire fraud. He could face up to 20 years in federal prison when he is sentenced.
Det Supt Bob Wishart, from the City of London Police's Economic Crime Directorate, led the investigation in partnership with US authorities.
"He and the guys who did this are on a par with some of the most ruthless villains out there," he told the Press Association.
Most of the victims were retired pensioners from the UK who trusted Pope with their life savings, Det Supt Wishart said.
"They did not deserve what they have got, they thought they were going to be able to look after their families for years to come with this."
One victim alone lost more than £800,000 ($1.29m), detectives said.
Six others were charged by US authorities alongside Pope in March 2009 and are awaiting trial. Two more Britons are among them.
Boiler room scams involve high pressure sales tactics used to con investors into buying non-tradable, overpriced or even non-existent shares.