Manchester United's bank balance has fallen by almost £100m in six months, according to the club's quarterly accounts, as a result of buying back bonds, player arrivals and stadium improvements.

The latest accounts show that while revenues continued to rise as a result of increased TV income and corporate hospitality revenue, plus new commercial contracts, higher wages contributed to sharply increased operating costs.

Overall revenues in the six months to the end of 2011 increased to £175m from £156.5m a year earlier, driven largely by rises in media and commercial income, including a new training kit deal with DHL. But operating costs rose to £110m from £96.9m.

The club will point to investment in refurbishing hospitality boxes, which has led to an increase in income, as evidence it can go on squaring the circle of shouldering debt interest repayments while continuing to invest in the squad and facilities.

The sharp fall in United's bank balance, from £150.6m to £50.9m, over the space of six months was largely a result of the £47m net outlay on transfers in the summer and £5.3m in the most recent quarter alone spent on buying back bonds. Protesters against the Glazers have long feared that the money in the account would be spent on further financial engineering rather than players.

The club, owned by Malcolm Glazer and his family, has now spent a total of £92.8m buying back bonds originally released as part of a contentious £512m bond issue in 2010.

Even though most of the repurchased bonds have been at a higher price than the initial sale value, it makes financial sense because it reduces the interest burden on the club. Despite the buybacks, the net interest paid during the last half of the year went up slightly to £24.5m but the club said this was due to when certain payments fell due compared with the previous year.

But supporters' groups, who have long protested against the Glazer model, will argue that money invested in buying back bonds is not being spent on the playing squad.

Since the sale of Cristiano Ronaldo in 2009 and a sponsorship deal with Aon that involved a substantial upfront payment, the Glazers have been able to point to United's healthy bank balance.

But the purchases of Ashley Young, David de Gea and Phil Jones have substantially reduced that cash pile, which stood at £164m at its height in June 2010.

The signings and new deals for existing players, together with an expansion in the club's commercial operation off the pitch, drove a sharp rise in staff costs. Wages rose to £38.7m during the final quarter of 2011, an increase of 17.3% compared with the previous year.

Despite the strong revenue growth, results in the second half of the year are likely to be affected by the club's elimination from the Champions League after the group stages. As a result, all of its Champions League income for the season has been booked in the first-half accounts and revenue from the Europa League will not compensate.

Detailed plans to float a stake of up to 30% on the Singapore stock exchange in a bid to realise $1bn in value have been put on indefinite hold due, according to club insiders, to the global economic uncertainty.

Manchester United Supporters' Trust said: "Manchester United revenues continue to grow strongly although costs are increasing just as quickly so pretty much negating that growth. However the key figures of interest to supporters show the Glazers have now spent every penny of the money received from the sale of Ronaldo, and more. That's now £92.8m spent on buying back their own bond debt that they loaded on to our club. So statements at the time that all of the Ronaldo money would be made available for reinvestment were clearly just spin."

"Since the sale of Ronaldo [in June 2009] net transfers [not including Ronaldo] have totalled just £90m while they have taken out of the club £225m to cover their debt payments and interest. What could the club have done with that extra £225m? Cheaper tickets for loyal fans, investing massively in the squad and stadium, developing and retaining the best youth players, competing on an equal basis with the very best teams in Europe. This is the true cost to Manchester United of the Glazers' ownership."