New rules to try to deal with the £25bn worth of money laundering which goes on in the UK have come into force.
The crackdown will also hit businesses who avoid paying their VAT.
Key professions like accountants and lawyers, but also casinos and jewellers too, will be duty bound to grass on their clients if they suspect them of using the proceeds of crime.
But don't panic - this will only involve transactions of more than £9,000 in cash.
Who will this affect?
The property trade, and estate agents in particular, will be at the frontline.
Because so many property transactions are done in cash, buying and selling houses is one way of making hot money look legitimate.
So property professionals, who have first sight of these transactions, will become the eyes and ears of the law.
From today, they will be obliged by law to report on any suspicions about money laundering or risk going to jail.
However there is concern that the authorities won't be able to cope.
The National Criminal Intelligence Service will be overwhelmed by zealous professionals making sure that they inform on anything that looks amiss.
Banks already struggle with thousands of reports each year of suspicious activities; this workload is set to double.
What they have to do
The first priority for any business will be to register with Customs and Excise.
Then they'll have to appoint a money laundering reporting officer; make staff aware of the rules and request identification from those wishing to pay with large quantities of cash.
But it's not just directed at organised criminals.
Petty tax avoiders will also be in the authorities' sights.
In some trades, such as small scale building, VAT avoidance is rife.
The industry federations are keen to stamp out the practice because law-abiding builders find it hard to compete but they're unclear as to what sort of cash payments will have to be scrutinised.
Barry Stephens is the chief executive of the National Federation of Builders. He accepts the new regulations but is rather to confused about what they actually entail.
"It could be a number of payments over a period that would exceed the £10,000 threshold but we're not clear as to whether individual payments below that level have to be reported or whether it's just single payments above that number," he says.
Complying with these regulations could be a burden on some small businesses but investigators say that in the pursuit of terrorists and criminal gangs, even information about a relatively minor transaction like thousands of pounds spent suspiciously on jewellery can help direct them to large scale money-laundering.