Manchester United's talks with online betting company Mansion have broken down.
Mansion chief negotiator David Kinsman said the two had verbally agreed a deal, but then United pulled out after talks with other firms.
United denied the claims, saying it never holds exclusive talks.
"We would understand why anybody would be disappointed at not securing the shirt sponsorship deal for Manchester United," the club said in a statement to the Press Association.
"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," it added.
The club is looking for a new sponsor to replace Vodafone, whose £9m-a-year deal ends at the end of the season.
Mr Kinsman said he and other Mansion executives had been "flabbergasted" by United's decision to pull out of negotiations on Sunday.
He added he had cut short a business trip in China to meet the club's commercial director Andy Anson after being led to believe the two had "done a deal".
"It was something of a surprise therefore to be told last night that United had been negotiating with someone else," he added.
"During the negotiating process we had no idea this kind of double dealing was going on. Manchester United have not acted as we expected them to."
The all-cash deal with Mansion would have involved a substantial up-front payment and been worth around £65m to £70m over the next four years.
Mansion chief negotiator David Kinsman said the two had verbally agreed a deal, but then United pulled out after talks with other firms.
United denied the claims, saying it never holds exclusive talks.
"We would understand why anybody would be disappointed at not securing the shirt sponsorship deal for Manchester United," the club said in a statement to the Press Association.
"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," it added.
The club is looking for a new sponsor to replace Vodafone, whose £9m-a-year deal ends at the end of the season.
Mr Kinsman said he and other Mansion executives had been "flabbergasted" by United's decision to pull out of negotiations on Sunday.
He added he had cut short a business trip in China to meet the club's commercial director Andy Anson after being led to believe the two had "done a deal".
"It was something of a surprise therefore to be told last night that United had been negotiating with someone else," he added.
"During the negotiating process we had no idea this kind of double dealing was going on. Manchester United have not acted as we expected them to."
The all-cash deal with Mansion would have involved a substantial up-front payment and been worth around £65m to £70m over the next four years.
