One of Manchester United's biggest backers has criticised the club for inflicting "reputational damage" in the wake of last month's foiled plan to establish a European Super League.

Nick Train, who manages the UK Equity Fund Lindsell Train, expressed alarm in face-to-face meetings with representatives of the controlling Glazer family as discontent over the proposals rumbles on.

Lindsell Train owns 27 per cent of United's New York-listed Class A shares, comprising around seven per cent of the club, while it also has a significant interest in Juventus, who are yet to extricate themselves from the breakaway, and Celtic, who were not involved.

Train wrote in his monthly report to investors: "The announcement of the breakaway European Super League ("ESL") in April came as a surprise to us.

"In response, we requested and have now had meetings with all three clubs (including Celtic, even though it was not a member of the ESL).

"At these meetings we expressed our disappointment about the reputational damage Juventus and Manchester United have inflicted on themselves.

"We asked for clarity about their position regarding the ESL going forward. Most importantly, we urged them to return to respectful negotiations with all members of the football community to work toward mutually beneficial ends.

"We continue to monitor events closely as they unfold, while considering their implications for our investment case."

Lindsell Train's concern echoes those of some of the club's commercial partners, including reports that kit sponsors Adidas were disappointed not to have been briefed in advance about the proposals.

United fans have vowed to continue their protests against the Glazer family when fans are allowed to return to Old Trafford for the club's final home match of the season against Fulham on Tuesday.

Earlier this month, United's game against Liverpool was postponed after fans invaded Old Trafford prior to the scheduled kick-off, while protests continued outside the stadium during this week's rearranged fixture.