LONDON—The man accused of being the ringleader of a global interest-rate-rigging conspiracy received a helping hand from his boss, according to evidence presented in a London trial Thursday.

Tom Hayes, a former UBS AG trader in Tokyo, got multiple assists from his direct manager, Mike Pieri, to get their UBS colleagues to move their interest-rate data to benefit Mr. Hayes's trading positions, according to emails shown in court.

Mr. Hayes, the first person to stand trial for allegedly manipulating the London interbank offered rate, or Libor, has pleaded not guilty. He previously told The Wall Street Journal that "this goes much, much higher than me." Mr. Pieri, who no longer works for UBS, hasn't been charged but is named as one of Mr. Hayes's alleged co-conspirators. He hasn't responded to requests for comment.

In a series of emails to a senior UBS investment-banking executive in London in October 2008, Mr. Pieri asked for dispensation to move UBS's Libor data slightly lower to suit Mr. Hayes's trades, noting that millions of dollars of potential profits or losses hung on the decision.

"We really need some cooperation on the yen Libors from those who input," Mr. Pieri wrote.

It isn't clear whether or how the recipient responded. But later that day, Oct. 9, Mr. Pieri sent a follow-up email, also to senior UBS executives, noting that he had "got some concession on this in the end. We will be a little bit lower. Every bit helps."

The exchange triggered some dissent within UBS. Another trader, Roger Darin, wrote to another UBS executive complaining about Mr. Pieri's efforts. "One of the things we signed up for when UBS agreed to join the [Libor] panel was the condition that fixing contributions shall be made regardless of trading positions," wrote Mr. Darin, who has been charged along with Mr. Hayes in the U.S. with manipulating Libor. Mr. Darin hasn't entered a plea but has sought to have the charges against him dismissed.

The fact that his boss and others knew what he was doing, and at times appeared to help, is likely to play a significant role in Mr. Hayes's defense, which will get under way later in the monthslong trial.

Mukul Chawla, the lead prosecutor on the case, previously has said that just because attempts at Libor-rigging were widespread isn't an excuse for Mr. Hayes's alleged conduct. "Because lots of people are doing it doesn't mean it's not fundamentally dishonest," Mr. Chawla said Wednesday.

Mr. Chawla on Thursday also presented evidence showing that Goldman Sachs Group Inc. tried to hire Mr. Hayes in mid-2008. Goldman offered to pay Mr. Hayes about $3 million, according to internal UBS emails at the time. To retain Mr. Hayes, UBS sent him a letter saying he was on track for a roughly $2.5 million bonus. That package was signed off on by the head of UBS investment bank.

The Goldman offer was previously reported by the Journal in 2013.

Mr. Hayes agreed to stay at UBS. But Mr. Chawla said that with the financial crisis raging, he never collected the bonus he had been told to expect.

Write to David Enrich at david.enrich@wsj.com

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