The Financial Conduct Authority (FCA) has fined Santander £32,817,800 for failing to effectively process the accounts and investments of deceased customers.
Santander did not transfer funds totalling more than £183m to beneficiaries when it should have done.
A total of 40,428 customers were directly affected.
Santander also failed to disclose information relating to the issues with the probate and bereavement process to the FCA after it became aware of them.
Mark Steward, executive director of enforcement and market oversight at the FCA, said the failings took too long to be identified and then far too long to be fixed.
He said: "To the firm's credit, once these problems were notified to the board and senior management, they were fixed properly and promptly. But recognition of the problem took too long.
"Firms must be able to identify and respond to problems more quickly especially when they are causing harm to customers.
"The FCA will continue to be on the lookout for firms with poor systems and controls and will take action to deter such failings to ensure customers are properly protected."
Santander breached principle three and principle six between January 1 2013 and July 11 2016 by failing to take reasonable care to organise and control its probate and bereavement process responsibly and effectively, with adequate risk management systems, and by failing to treat its customers and those who represented them on their death fairly.
Santander's probate and bereavement process contained weaknesses which reduced its ability to effectively identify all the funds it held which formed part of a deceased customer’s estate.
The failings also resulted in Santander failing to effectively follow-up on communications with deceased customer representatives which increased the likelihood of probate and bereavement cases not being closed.
The FCA also ruled the bank ineffectively monitoring open probate and bereavement cases and therefore wasn't able to determine whether cases had progressed to closure.
These weaknesses meant the probate and bereavement process would start but it would stall and remain incomplete, meaning that funds would not be transferred to those who were entitled to them despite Santander being informed that a customer had died.
Certain funds belonging to deceased customers were also found by the FCA not be identified and transferred to those who were entitled to them who were unaware of the existence of those funds.
In some cases, funds were held for many years contributing to beneficiaries being deprived of the use of them for a considerable amount of time.
In addition, the FCA stated Santander took too long to address the issues within its probate and bereavement process, once it became aware of them, and to commence remediation exercises to transfer funds from affected accounts to beneficiaries.
Since 2015, Santander has carried out remediation exercises, to transfer funds from affected accounts to beneficiaries.
According to the FCA these exercises are almost complete and where possible Santander has located beneficiaries and transferred funds to them (or is in the process of doing so).