Anti-money laundering rules, which came into force on Monday, will force brokers to put clients purchasing items worth over £10,000 with cash to go through extra security checks.
The extension of the Proceeds of Crime Act will now cover more sectors, including brokers and estate agents. Tougher rules are being introduced throughout Europe.
Professionals who deal with company finances could be jailed for up to five years if they fail to report anomalies. But they are not allowed to tip off clients they have reported.
The National Criminal Intelligence Service (NCIS) said it estimated that up to £25bn is laundered through the UK each year.
City law firm Legal Risk warned the new rules would affect city firms, with many larger companies unaware they could be at risk from prosecution.