The National Crime Agency has literally no idea how much money is being laundered in London
London - a hub of international banks, lawyers, and accountants - attracts a lot of dirty money.
"High-end money laundering is a big focus for us going forward," David Little, head of money laundering at the National Crime Agency (NCA) said in a speech on Thursday.
The NCA leads UK law enforcement's fight against serious and organised crime.
"The people that are setting these structures up are very clever and have access to highly-skilled people," Little said.
"We don't know what the true amount is but it's in the tens or hundreds of billions. It's a truly terrifying number," Little added.
Britain doesn't have long to stem the billions being washed clean in the City.
The Financial Action Task Force, an independent international body tasked with stopping money laundering and terrorist financing, will give an assessment of the UK's anti-money laundering protocols in 2018.
The report will be an influential one, and potentially embarrassing for Prime Minister David Cameron and Chancellor George Osborne if it proves critical of the UK's approach.
"The UK is very attractive place to do business, both legitimate business and illegitimate business," said Little.
"The barriers to entry for illegitimate business aren't high enough. It's very important we have a credible deterrent to stop people laundering money. We're some way off having that."