Citi says defective software dating from the 1990s was the cause of a $1 billion wire transfer screw up earlier this month.

Citi is running multiple lawsuits to recover funds sent in error to creditors of the struggling cosmetics company Revlon.

According to Bloomberg, Citi was midway through a project to replace the obsolete software when a 'clerical error' - ie manual intervention was required - resulted in the transfer of the full $900 million debt repayment, rather than the interest that was due on the loans.

“We take pride in the role that we play as a global leader in financial services and recognise that an operational error of this nature is unacceptable,” Citigroup says in a statement. “We have put significant, additional controls in place until the new system is operational.”