Internet gaming group Mansion said that the two had verbally agreed a 拢70m shirt sponsorship deal following the termination of the Premier League football team's existing contract with Vodafone, but United broke talks off at the weekend and revealed it was in talks with other firms.
The football club is understood to have now agreed a final deal worth more than 拢15m a year with one of two Middle Eastern airlines, Etihad Airways or Qatar Airways, which the club had been reported to be in talks with before Mansion came on the scene.
The club has slammed accusations, claiming that "all the parties were aware several discussions were going on".
David Kinsmann, Mansion chief negotiator, said that he and other Mansion executives had been "flabbergasted" by United's decision to pull out of negotiations on Sunday.
"It was something of a surprise to be told that United had been negotiating with someone else.
"During the negotiation process, we had no idea this kind of double dealing was going on. Manchester United has not acted as we expected them to," he said.
Manchester United has confirmed that talks about a sponsorship deal with Mansion were terminated over the weekend.
A spokesperson for the club said it understood why Mansion was disappointed at not securing what he called the best shirt sponsorship deal in sport.
"We have not and do not enter into periods of exclusive negotiation and every party we have discussed the shirt sponsorship with are aware of that.
"All the parties were aware that several discussions were going on at the same time and our priority is to look for the most appropriate deal for Manchester United. We're sorry that Mansion feel the way they do but we wish them well in the future."
United's deal with the current shirt sponsor Vodafone ends in May.
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