A civil lawsuit claims that David Sachman, finance director for the company until his resignation earlier this year, is in reality Paul Yorke- Wade, business associate of Alan Teale, America's best-known insurance fraudster.
Mr Yorke-Wade was arrested earlier this year in France pending extradition to Belgium for questioning by Antwerp police over his alleged role in the pounds 150m Dai-Ichi Kyoto insurance fraud.
The former Lloyds broker, who ran Teale's Victoria Insurance Company in the US until it became the subject of a congressional fraud inquiry, is named in writs filed against Equisure by the Texas insurance agency RISC International.
The agency accepted a $650,000 (pounds 417,000) settlement from Equisure last week after accusing Equisure of fraud, misrepresentation and failure to honour agreements. It claims that Equisure hid Mr Yorke-Wade's role and misled it about the identity of David Sachman.
According to court documents, officers from RISC travelled to Equisure's offices after Mr Yorke-Wade's arrest to meet Mr Sachman, only to find him absent. Equisure offered a series of excuses for his disappearance but did not disclose that Mr Sachman was Mr Yorke-Wade, or that he was not available because he was in prison, the company alleges.
Peter Uttley, Equisure's chairman, said that Mr Sachman had resigned in May and was uncontactable in South Africa. He declined to comment on RISC's claims. But a former senior Equisure employee has confirmed that Mr Yorke-Wade did play a key role in Equisure. He says Mr Yorke- Wade was the power behind Equisure's London insurance intermediary, Hunter La Roche. According to the former employee, one Lloyd's broker was so concerned about Mr Yorke-Wade's involvement that it demanded Equisure sign a letter distancing itself from Hunter La Roche.
Documents filed in the US show that Mr Yorke-Wade's alleged alter ego, David Sachman, claims to be a former manager of Krediet Bank in South Africa. Investigators have been unable to find any trace of the company. The similarly-titled Belgian bank has confirmed that Mr Sachman has never worked for its South African office.
Meanwhile doubts have arisen over the identity of another of Equisure's British directors, Peter d'Orleans-Clarke, who took over as chief executive of Equisure last week.
In a deposition produced for the RISC case, Mr d'Orleans-Clarke claimed to have an intermediate law degree from the University of London and to be a partner with an "EEC consulting firm".
The University of London said Mr d'Orleans-Clarke has never been registered there as a student, nor has it ever offered intermediate law degrees.
Equisure's Essex-based freelance accountant Stephen Muggleton's premises were raided by agents for the US Stock Exchange earlier this month. He also acted as the accountant for the fraudulent Dai-Ichi group of companies before their spectacular collapse in 1995.
Equisure said last week it is to voluntarily withdraw from the Stock Exchange rather than face compulsory de-listing as investigations continue.