The 35-year-old former yen derivatives trader told the court it was wrong to assume he had been qualified to answer some of the questions he had responded to after his arrest in England in December 2012.
"What I said in 2013 relating to events in 2006 -- there were so many mistakes," he told the prosecution, led by senior counsel Mukul Chawla. He also said that his memory of events such a long time ago was often hazy. "You're treating me as if I know everything about everything and I don't," he told the court.
Hayes is the first trader to stand trial over allegations he rigged the London interbank offered rate (Libor), against which around $450 trillion of financial contracts are pegged worldwide.
The prosecution alleges that Hayes set up a network of brokers and traders spanning key financial institutions to defraud others by manipulating Libor to boost profits and bonuses.
In the months following his arrest in 2012, Hayes was questioned in 82 hours of interviews with SFO investigators, during which he made what prosecutor Chawla has described in court as "full confessions" about Libor manipulation.
Hayes has told the court he was initially driven to cooperate with the SFO by his fear of being extradited to the United States, where he is also wanted on fraud-related charges. He wanted to be charged in Britain, where his family, wife and child live.
He eventually pulled out of the SFO agreement to plead not guilty to eight counts of conspiracy to defraud between 2006 and 2010.
Hayes has told the court that he was open about his attempts to influence yen Libor rates -- designed to reflect interbank borrowing costs -- that his managers were aware of his trading methods and that the practice was widespread in the industry at the time.
Hayes, who has been diagnosed with mild Asperger's Syndrome, was shown a summary of a February 2013 SFO interview in which he was asked which bank was the most honest rate submitter. His response at the time, which was shown to the court, was that it would probably be "someone like a Chase (JP Morgan) ... or a Royal Bank of Canada (RBC)".
But when repeatedly asked by the prosecution to confirm this on Wednesday, Hayes said he was not really qualified to make such a statement.
"A large part of these interviews were effectively educated guesses," he told the court.
Hayes, who worked for UBS (>> UBS Group AG) and Citigroup (>> Citigroup Inc) in Tokyo between 2006 and 2010, also repeatedly urged the prosecution to interview former colleagues and stop asking him to speculate about who knew what when.
"It doesn't really matter what I recollect," he said. "I'm not qualified to answer the question."
"Not qualified, or not willing?" Chawla asked him.
"Not qualified," Hayes responded.
Hayes also denied that a request he made to a UBS Libor submitter in a December 2006 computer chat shown to the court to "skew the Libors a bit," was a reference to manipulation. He said it was a mathematical term that referred to asymmetry.
(Reporting by Kirstin Ridley. Editing by Jane Merriman)
By Kirstin Ridley