When the Turkish entrepreneur Selman Turk launched a new digital bank three years ago, he was entering a crowded market of online start-ups hoping to grab business from the traditional banks.

It seemed there was little to distinguish the new London-based venture, Heyman AI, from a host of other contenders. It had a rudimentary website, meagre funding compared to some of its digital rivals and no banking licence. “We are building a good bank for good people,” said its website. A footnote added that it was “in the application process” to become a bank in the UK.

But just seven months after the new venture was launched, a besuited Turk was on stage with the Duke of York at St James’s Palace collecting a people’s choice award in the prestigious Pitch@Palace event. The competition on 6 November 2019 was Prince Andrew’s version of the BBC show Dragons’ Den, but with a priceless royal cachet. It was a spectacular coup for former Goldman Sachs banker Turk and his fledgling bank, but did not secure the future of the firm, which was wound up in September last year.

It emerged last week that Turk, 35, is being sued by a Turkish millionairess, Nebahat Evyap Isbilen, 77, who claims she was defrauded by him. She is seeking the return of about £38m. He disputes the allegations.

Turk helped manage Isbilen’s financial affairs, and the court documents claim Turk instigated a payment of £750,000 to be made to Prince Andrew, just nine days after he picked up the award at St James’s Palace.

The money, from Isbilen’s private bank account, has since been repaid, but raises new questions for Prince Andrew: were he and his advisers aware the payment was linked to Turk, and what was the money for? Prince Andrew is not central to the complex high court proceedings and is not accused of any wrongdoing.

In her statements to the high court, Isbilen said she was told by Turk that the payment was a gift for Prince Andrew after it was claimed he helped arrange a replacement passport.

She also said she had seen an email from Turk to her private bank, Hampden & Co, stating the payment was a wedding gift for Princess Beatrice, who married the property tycoon Edoardo Mapelli Mozzi in July 2020.

Isbilen has told the court she considered the claims concerning Prince Andrew’s purported role in arranging a passport or that the money transfer was a wedding gift were both false.

In an extraordinary transcript of a call published by the Daily Mail yesterday, Amanda Thirsk, Prince Andrew’s former private secretary and the former director of Pitch@Palace, allegedly told a banker from Hampden & Co conducting compliance checks that she believed the money was a gift. She is claimed to have said: “I understand it’s a gift for the wedding…What she [Beatrice] and her family decide to do with it is really to do with them, isn’t it?” Thirsk told the Mail she could not recollect the call, but would not have been involved in anything improper.

It has also been reported that Prince Andrew’s former wife, Sarah Ferguson, also received payments of at least £225,000 from Turk via a company called Alphabet Capital, said to be in connection with payment she was owed for her work with a solar power company. Her spokesperson has said she was completely unaware of the allegations which have now emerged.

Isbilen is a member of the Turkish family that owns the Evyap group of companies, which has a range of soap and personal care brands. Her husband, Ilhan Isbilen, is the former deputy leader of the ruling AK party, and has been a political prisoner in Turkey since 2015.

Isbilen says she gave Turk effective control of funds of $87.5m (£66.7m) and that he “dishonestly and systematically” abused the trust she placed in him. She said she had limited ability to understand English and required help to move her assets out of Turkey. She claims he dishonestly mismanaged her funds.

Pitch@Palace was founded by the Duke of York in 2014 to “amplify and accelerate the work of entrepreneurs”. The programme had aimed to help businesspeople “make connections” and secure investment.

Its future was, however, in peril at the time Turk picked up the award for his bank.

Prince Andrew had given his notorious BBC Two Newsnight interview 10 days after the awards ceremony, triggering a public backlash. Companies including KPMG, Barclays, Bosch and Standard Chartered subsequently ended their sponsorship of Pitch@Palace. Its future activities are now under review.

Prince Andrew’s spokesperson has said there will be no comment on an ongoing court case.