The two per cent hike will see the club pay millions extra on a payment-in-kind (Pik) loan of more than £200m, which is due to be repaid by 2017.
The PIK loan will total more than £267m at the end of June next year due to the new rate of interest, rising to more than £310m by 2012.
The interest payments must be made by the Glazer family rather than the club itself, but fans’ groups now fear the club’s already precarious finances will be further affected.
Manchester United Supporters’ Club (Must) chief Duncan Drasdo said: “The rise in the Pik interest rate this week, caused by the Glazers’ inability to hit their own targets, is another sad chapter in the story of their exploitation of Manchester United.
“The increasing cost of the Piks will inevitably be borne by Manchester United. The Glazer family have failed to repay a single penny of their debts in the last four years... and the only source of cash is our football club.”
The Glazers tied around £716m of debt to the Premier League outfit when they bought it in 2007.
A group of powerful City figures, led by Goldman’s chief economist Jim O’Neill, launched a bid to wrest control of the club but the Glazers have been unwilling to do a deal.