Manchester United have converted last year's record losses into a £29.7m pre-tax profit, reflecting on-field success and strong commercial growth, and reduced their net debt levels. But their controversial owners, the Glazer family, will continue to face criticism for the effects of their leveraged business model after £51.7m flowed out of the club in interest payments alone.

The results show revenues rose to a record £331.3m in the year ending June 2011, an increase of 16% on the previous year's total, as Sir Alex Ferguson's side won the Premier League and reached the European Cup final.

Ahead of next month's planned flotation of a minority stake in Singapore, the club delivered a record operating profit (before interest, amortisation and exceptional items) of £110.9m, a figure that topped £100m for the sixth year in a row. But United paid out £51.7m in interest payments on the £526m bond scheme that replaced bank loans in 2010. The club spent almost £64m buying back some of those bonds in the past year.

In 2009-10, the club posted pre-tax losses of £79.2m, largely as a result of write-downs and one-off charges relating to the bond issue. They have now shifted to a different accounting standard, using IFRS rather than the UK GAAP standard, under which that loss would have been £14.9m.

The figures show a sharp rise in overall costs, up to £220.5m from £185.2m the previous year, including a 16.1% increase in staff costs as a result of increased player wages and bonuses. However, the crucial wages-to-turnover ration remained at a healthy 46%.

Those who have campaigned long and hard against the Glazers' ownership model will point to the £51.7m paid in interest as evidence that is still acting as a drag on the club's finances.

Andy Green, who blogs on United's finances as Andersred, has calculated that a total of £478m has been spent on servicing debt at the club since 2005. That figure comprises £373m in interest and fees and £105m repaying debt. But United executives are likely to point to continued success on the field and strong growth. Net debt stands at its lowest level since the Glazers bought the club in a £790m leveraged deal in 2005. With £150.6m cash in the bank at the end of the financial year, and £63.8m of debt repurchased during the year, the net debt stood at £308.3m at the end of June.

The amount in the bank will now have decreased by around £50m after the signings of David de Gea, Ashley Young and Phil Jones. Upward pressure on wages, in part due to Wayne Rooney's £180,000-a-week deal, and bonuses paid to players, saw staff costs rise by 16%. However, since the accounts were filed, several high earners have been moved off the wage bill.

"It's a continued improvement year after year, but that improvement has been driven by Ferguson and the players," the United Supporters Trust chief executive Duncan Drasdo said. "The growth in revenue is positive, but it's what happens to that income. Does it stay in the club or go out in interest, bond repayments or payments to the owners? That's why the Initial Public Offering is potentially such an important opportunity. We want to see the debt cleared once and for all."

Sources close to the float have promised that the proceeds will be used to pay down debt and boost commercial operations in Asia.