Santander has been fined £107.8million by the City watchdog over what it called “serious and persistent” anti-money laundering failures.
The Financial Conduct Authority (FCA) the bank "failed to properly oversee and manage" systems aimed at verifying information.
The bank's oversight of more than 560,000 business banking customers was affected between December 2012 and October 2017.
The failures led to more than £298million passing through the bank before it closed accounts.
In one case, a new customer opened an account as a small translations business with expected monthly deposits of £5,000.
But within six months, the account was receiving millions in deposits and swiftly transferred the funds to separate accounts.
The FCA said the particular account was recommended for closure in March 2014 but was not acted on until September 2015 due to "poor processes".
Mark Steward, executive director of enforcement and market oversight at the FCA, said: "Santander's poor management of their anti-money laundering systems and their inadequate attempts to address the problems created a prolonged and severe risk of money laundering and financial crime.
"As part of our commitment to prevent and reduce financial crime, we continue to take action against firms which fail to operate proper anti-money laundering controls."
Santander chief executive officer Mike Regnier said: "Santander takes its responsibilities regarding financial crime extremely seriously.
"We are very sorry for the historical anti-money laundering (AML) related controls issues in our Business Banking division between 2012-17 highlighted in the FCA's findings.
"While we took action to address our AML issues once they were identified, we accept that our AML framework at the time should have been stronger.
"We have since made significant changes to address this by overhauling our financial crime technology, systems and processes.
"Today over 4,400 staff are focused on preventing financial crime and we continue to invest to meet our responsibilities and keep our customers and communities safe."
Santander has been included in our full list of bank branches set for closure in 2023.
HSBC is to shutter 114 banks - around a quarter of its total sites - in 2023.
Lloyds is closing a further 14 banks next year, while NatWest currently has 43 scheduled to shut their doors forever.
Barclays will shut 11 sites in 2023, Santander and Halifax will shutter five each and Nationwide will close one.
