NatWest faces criminal action over allegations it failed to detect suspicious activity.
Britain's financial regulator says a customer deposited around 553 million dollars over five years, much of it in cash.
Now the Financial Conduct Authority has begun the first criminal action against a UK bank under a 2007 money laundering law.
There is no limit on the fine the firm could face.
The FCA says the suspicious transactions ran until October 2016.
Sources say the customer was a gold dealer based in northern England.
It was liquidated after a police raid at around the time the transactions stopped.
NatWest will now appear in court on April 14.
A source told Reuters that individuals could also be charged.
NatWest disclosed in its 2020 annual report that it faced an investigation.
The case threatens it with new fines for past misdeeds, just as it is trying to clean up its image.
NatWest rebranded from the scandal-tainted Royal Bank of Scotland name last year.
Shares in the lender were down around 1% by lunchtime on Tuesday (March 16).