The Swiss bank's US brokerage unit violated Regulation SHO, which requires a broker-dealer to have reasonable grounds to believe that the security could be borrowed and available for delivery before accepting or effecting a short sale order, says Finra.

The rules require firms to obtain and document this "locate" information before the short sale occurs to help cut the number of potential failures to deliver.

However, UBS' Reg SHO supervisory system for locating and the marking sale orders was "significantly flawed and resulted in a systemic supervisory failure that contributed to serious Reg SHO failures across its equities trading business".

The brokerage placed millions of short sale orders to the market without locates, including in securities that were known to be hard to borrow. The violations extended to "numerous trading systems, desks, accounts and strategies, and impacted UBS' technology, operations, and supervisory systems and procedures".

In addition, UBS mismarked millions of sale orders in its trading systems. Many of these were short sales that were tagged as "long". Finra also found "significant deficiencies" related to aggregation units.

Brad Bennett, chief of enforcement, Finra, says: "Firms must ensure their trading and supervisory systems are designed to prevent the release of short sale orders without valid locates, and properly mark sale orders, in order to prevent potentially abusive naked short selling. The duration, scope and volume of UBS' locate and order-marking violations created a potential for harm to the integrity of the market."

UBS has neither admitted nor denied the charges in consenting to Finra's findings.

After a tough summer in which it emerged that it had suffered a $2.3 billion loss through unauthorised trading, yesterday the bank admitted in a document to the SEC that internal controls were "not effective".