Britain's financial watchdog has fined Bank of New York Mellon 126 million pounds ($186 million) for breaking rules meant to protect customer assets.

The Financial Conduct Authority said Wednesday it fined the bank's London branch and Bank of New York Mellon International Limited for failings between 2007 and 2013.

During that time, the bank — the world's largest custody bank — held over 1 trillion pounds' worth of assets from over 6,000 Britain-based customers. A custody bank has an obligation to secure assets for safekeeping.

But the watchdog said the bank failed to properly record and protect assets, and that it failed to take steps to prevent them from being mixed up with other accounts.

It said those failings were made more serious because they came during the financial crisis.