Tim Leissner, who had close ties to the Malaysian sovereign wealth fund 1MDB and was considered Goldman Sach's point man in Singapore dealing with the fund, was interviewed on January 19 this year in relation to "inaccurate and unauthorised statements" made by him in a June 2015 reference letter.

According to filings with the Financial Industry Regulatory Authority, the US securities industry's self-regulating body, the letter was written without Goldman Sach's knowledge or approval.

Sources close to the case said Leissner's letter had included details about the finances of his client, who is believed to be well-known Malaysian tycoon Jho Low, while overstating the extent to which Goldman Sachs had done due diligence on him.

The letter, which vouched for Low and his finances to a financial institution in Europe, was to be used to help Low reach a deal to buy real estate from the National Asset Management Agency, it is believed.

A day after Goldman Sachs quizzed Leissner over the unauthorised assurance letter, he tendered his resignation with immediate effect. It is not clear if Low progressed any deal with Nama on foot of the letter.

Based in Singapore, Leissner had been a chief advisor to the state investment fund 1Malaysia Development Berhad, or 1MDB, that netted Goldman Sachs hundreds of millions of dollars in commissions for bond sales. The investment bank earned an estimated $593m in commissions and expenses from three bond sales totalling about $6.5bn since 2009.

The fund is one of the biggest investment groups in the world, with advisers including French billionaire Bernard Arnault.

Goldman has severed ties with the controversial fund that is now the subject of a $6bn international embezzlement investigation covering five countries including France, Switzerland and the UK, and allegations that $681m was routed to private bank accounts owned by Malaysia's Prime Minister Najib Razak.

Mr Razak has denied accusations of corruption, which have dogged his government for months. US investigators probing the troubled Malaysian state investment fund have subpoenaed Leissner, Goldman Sachs' former chairman for Southeast Asia. He denies any wrongdoing.

1MDB has said all its investments are accounted and that it would co-operate with any investigations.

Goldman Sachs hasn't been accused of any wrongdoing.

Billionaire tycoon Jho Low, who helped set up 1MDB, has strongly denied that he benefited from alleged improprieties linked to the fund.

Low is well-known in Irish property circles after his Wynton Group firm became embroiled in the bitter and highly public boardroom battle that broke out between Paddy McKillen and the Barclay Brothers for control of the landmark Claridge's, Connaught and Berkeley hotels in London.

The Malaysian-based investor, with backing from 1MDB, approached Nama over buying Derek Quinlan's loans.

According to court filings in the subsequent court action between McKillen and the Barclays, Low "acquired a debt due from Mr Quinlan to Nama in April 2011 which was secured on part of Mr Quinlan's shareholding in the company, bidding more for the debt than the Barclay interests were prepared to pay. The debt was acquired through an associate company called JQ2".

A letter from Wynton Group to shareholders in the hotel group in January 2011 stated that financing for the offer had "in principle been fully underwritten by Malaysian government-backed investment funds".

However, media reports at the time stated that Nama was initially reluctant to engage with 1MDB despite the size of its coffers.