UEFA is considering changes to its financial fair play regulations which could cause problems for Manchester United.
European football's governing body has penalised clubs that made losses, including Manchester City and Paris Saint-Germain, who were both fined 40 million pounds this summer.
Now they may opt to switch their attention to those that are in debt -- including United -- at a meeting on the issue next week.
UEFA general secretary Gianni Infantino said: "We're now focused on losses, and to repay the debt is part of the loss that the club can make at the end of the season. But, certainly, the question of debt is something that can be put on the table."
United owe around 350 million pounds, down from over 700 million in 2010, with the figure representing less than a year's turnover at Old Trafford.
#INSERT type:image caption:United were free of debt before the Glazers took over in 2005. END#
The club were free of debt before the Glazer family's takeover in 2005, but the Americans financed the buyout by borrowing against the club and around 700 million pounds has been spent servicing the debt in the subsequent nine years.
Real Madrid, more than 600 million euros in debt, are another club who could suffer under the proposed changes.
Manchester City complained in May when they were found guilty of breaching FFP rules despite being debt-free, and said they would be profitable by next year.
Chairman Khaldoon al Mubarak told the club's official website: "We have a sustainable project today, in Manchester and across the board in the City Football Group.
"We have zero debt. We don't pay a penny to service any debt. For me, that is a sustainable model. However, our friends at UEFA seem to believe otherwise. They have their view, we have ours."