US banking giant Bank of New York Mellon has been fined £126 million for breaking rules designed to protect clients' assets, British regulators said Wednesday.

The penalty, equivalent to $186 million or 175 million euros, was handed down by London's Financial Conduct Authority watchdog over failings at the group between November 2007 and August 2013.

The FCA said in a statement that the bank was fined for breaching rules in relation to its role as a so-called "custody bank" with the systematically important role of keeping assets safe.

The failings covered both the Bank of New York Mellon London Branch and the Bank of New York Mellon International Limited.

The two divisions held safe custody asset balances peaking at £1.3 trillion and £236 billion respectively during the period.

“Our custody rules are in place to ensure that clients are protected in the event of insolvency," said Georgina Philippou, acting director of enforcement and market oversight at the FCA, in the statement.

"The firms’ failure to comply with our rules including their failure to adequately record, reconcile and protect safe custody assets was particularly serious given the systemically important nature of the firms and the fact that safeguarding assets is core to their business.

"Had the firms become insolvent, the total value of safe custody assets at risk would have been significant. This is compounded by the fact that the breaches took place at a time when there was considerable stress in the market."

The FCA added that BNY Mellon had agreed to settle at an early stage of its probe. That qualified the group for a 30-percent discount from the fine which would otherwise have been £180 million.

In a separate statement, BNY Mellon said it has "worked cooperatively" with the FCA to "address issues" related to its compliance with the rules.

The US lender added it had launched a "broad" internal review and taken "clear steps" to reinforce its compliance.