Manchester United have been forced to place 4,000 Old Trafford season tickets on general sale.

Earlier this week United said 50,000 season tickets had been sold, a milestone seen as a victory for the Glazer family against protesting fans.

The "Green and Gold" protest movement - unhappy at the club's debt level under its American owners - became a visible presence at Old Trafford last season.

Fans' groups say United has suffered a marked fall in year-on-year sales.

Manchester United sold 64,000 season tickets two years ago, of which 8,000 were all-important premium tickets, and only 500 fewer last season.

One close watcher of United's finances, who wished to remain anonymous, said sales were thought to be "5,000-10,000 down on last year".

He said this was down to discontent with the Glazer regime and the wider economic situation.

It is unclear whether there is a link between the fan discontent and the issue of season ticket sales. Numerous attempts have been made to reach Manchester United for an official comment this summer but none have been successful.

It is also unclear how many of the club's 8,500 premium seats - 7,000 executive seats and 1,500 hospitality packages - have been sold.

Over 40% of Manchester United's match-day revenue comes from these premium seats, despite them making up only 11% of Old Trafford's 76,000 capacity.

The need to place season tickets on general sale - seven weeks after the deadline for renewals had passed - also raises questions about the club's much-trumpeted waiting list, an untapped reservoir of support supposed to number 60,000 fans.

This is an issue that is likely to come up with investors in August when the next set of quarterly accounts are due.

The three-monthly reporting of the club's financial results was a condition of this year's controversial £504m bond issue and chief executive David Gill could experience an uncomfortable bondholders' conference call.

But nobody can accuse the Premier League giants of not trying hard enough to shift their tickets.

Anecdotal evidence suggests there has been an unprecedented sales drive this summer, with particularly attractive deals offered to wavering executive-seat customers.

The backdrop to this story is one of booming ticket sales at Manchester City - albeit from a much lower base - and solid sales performances from rivals such as Arsenal.

In June the Red Knights group of wealthy supporters mulling a takeover of United, which was backed by the "Green and Gold" protest movement - put plans to make a bid for the club on hold.