Insurance company Aviva has been fined £8.2m by the Financial Conduct Authority (FCA) for breaking the rules on the protection of clients' savings.
The problem occurred when administration was outsourced to a platform used by financial advisers between 2013 and 2015.
No individuals suffered a loss.
The company apologised, and said clients' money - including pensions and investments - was now properly protected.
The FCA said Aviva had failed to put in place appropriate controls that would ensure investors would get their money back, should the company fail.
"Had Aviva suffered an insolvency event during the period, customers could have suffered loss," it said.
"Other firms with similar outsourcing arrangements should take this as a warning that there is no excuse for not having robust controls and oversight systems in place to ensure their processes comply with our rules," said Mark Steward, director of enforcement and market oversight at the FCA.
'We are sorry'
In response Aviva, one of the largest pension providers in the UK, said it had strengthened its controls.
It has also established a specialist team to oversee the safety of customers' money.
"This should not have happened and we are sorry," said Andy Briggs, chief executive of Aviva UK Life.
"Aviva's customers have not suffered any loss and there has been no impact on advisers. We have addressed and resolved the issues identified."