Singapore-based businessman Oei Hong Leong has sued Citigroup's private bank for negligence and misrepresentation after he lost S$1 billion ($684 million) on foreign exchange and bond trades last year, the Straits Times reports.

Mr Oei's lawsuit said Citi, with which he has a 30-year relationship, repeatedly gave him an inaccurate picture of his trading exposure, which led him to take on more positions than he would have taken otherwise, the newspaper said, citing court documents. It said Mr Oei declined to comment about the court case.

"We have acted professionally in managing Mr Oei's account. We have comprehensive and robust systems and processes in place to handle client orders and transactions and for monitoring and control. Beyond this it would not be appropriate for us to comment further at this stage as the matter is now before the Courts,” a spokesperson told WealthBriefing.

“It is therefore proper that our response to the allegations be found in court documents to be filed at the appropriate juncture. We fully intend to defend our position vigorously,” the spokesperson added.

Mr Oei was ranked Singapore's 29th richest person by Forbes last year with a net worth of $210 million. Forbes calculated Mr Oei's wealth based on his stakes in publicly traded companies and in private company filings.

According to Straits Times, Mr Oei claimed he felt compelled to close his positions at an extremely volatile time in October last year, taking huge losses, as he felt he had no choice after discovering the full extent of his exposure.