This is the first time it has been officially suggested that the former head of copper trading profited personally from his fraud and the first time a firm in Britain with links to Mr Hamanaka has been alleged in a criminal court to have been involved in misconduct.
Charles Vincent and Ashley Levitt, Winchester's wealthy founders who now live in Monte Carlo, have strenuously denied involvement in irregular dealing with Mr Hamanaka.
They have also maintained Sumitomo board members approved a complex copper deal with Mr Hamanaka, codenamed RADR, but both prosecution and defence agreed in court that Mr Hamanaka had been acting without the knowledge of anyone else in Sumitomo.
Mr Hamanaka was pleading guilty yesterday to fraud and forgery, in a trial that promises to answer very few of the outstanding questions about one of the world's biggest financial scandals. Charges of breach of trust were dropped.
The prosecution alleged that Mr Hamanaka received 15m (pounds 75,000) in cash from Shinichi Nishi, the Tokyo representative of Winchester, who was a close acquaintance of his.
Mr Hamanaka used the money to entertain clients and others at nightclubs, to buy golf club memberships and to make overseas trips.
In court Mr Hamanaka quietly answered, "That is correct," to a string of charges relating to 10 years of unauthorised trading which depressed global copper prices and led to international fraud investigations in Britain and the US.
The revelations in the Tokyo court come against the background of a Serious Fraud Office criminal investigation of the London connections in the copper scandal and a separate Securities and Investments Board (SIB) inquiry, both of which are continuing.
The 44-page prosecution statement accused Mr Hamanaka of forging four letters to open accounts, and fraudulently diverting $771m in fake copper warrant deals from Sumitomo's Hong Kong office to an account run by Morgan Guaranty, a New York subsidiary of JP Morgan.
The maximum sentence for the combined charges is 15 years, although Mr Hamanaka's guilty plea and willingness to co-operate with the authorities is likely to mitigate this.
Although the defence does not dispute the charges, it will argue that inadequate risk management contributed to his offences. In court, however, both sides agreed that he acted alone. "Through various deceptions, he pretended that Sumitomo's copper trading team always turned a profit and that he was a talented dealer," the prosecution alleged yesterday. "Therefore, he had the full confidence of his superiors."
The Securities and Investments Board angrily denied a claim by Panorama reporters that Martin Vile, then capital markets director at the SIB, urged the LME not to investigate allegations made by David Threlkeld, a well-known metals trader. Mr Threlkeld wrote to the regulators in 1991 alleging Mr Hamanaka had asked him to confirm several fictitious trades. A SIB spokesman said: "On 10 December 1991, four days after the letter was written, Mr Hamanaka and another Sumitomo official were interviewed jointly by the SIB and the LME. We believed that Sumitomo Corporation at that time endorsed the explanations given."