Turnover at the Ibrox club dropped by 37% last year
Rangers Football Club made a loss of nearly £13m last year, compared with a profit of £7.2m the previous year.
The extent of the club's financial woes was revealed in their annual report, which showed total debts rising from £21m to more than £31m.
Turnover at Ibrox fell from £64.5m to £39.7m in the past year.
Rangers are currently seeking new owners, with former chairman Sir David Murray keen to off-load his controlling stake.
Details of the financial situation at the club were revealed on the same day the English Premier League rejected a proposal to allow Celtic and Rangers to join the English top flight.
The Old Firm pair had hoped to be part of new plans put forward by Bolton chairman Phil Gartside for a two-tier league.
By Douglas Fraser, BBC Scotland Business and Economics Editor
A £31m debt for Rangers is big, but not necessarily unsustainable.
The problem is how fast it's increased - by £10m since last year - and how willing Lloyds Banking Group is to keep bankrolling the club.
Of perhaps more significance is the sharp decline in turnover - a 38% drop to just below £40m.
The roller-coaster of football fortunes would be tough for any other company to handle, but this one's problems are crowding in: it's failing to live up to its European aspirations, its kit distribution deal with JJB is "unsatisfactory", and the Setanta TV deal fell apart, leaving it with half its expected TV rights earnings.
After a bumper year in Europe, tickets and hospitality earnings were down by a third in this annual report, commercial earnings by two-thirds, yet the dominant wage bill has fallen less than £4m to a still daunting £30m.
Compare that turnover at £40m with the revenue reported for the same period for Celtic, at £73m.
Financially, that makes it look like the rival clubs are in different leagues.
New Rangers chairman Alistair Johnston said the club's operating costs could not be sustained without European competition, and that the club had been operating in "the most challenging economic climate for many years".
Chief executive Martin Bain said it was necessary to look for new sources of income, including talks "with other football clubs in the continent facing similar challenges".
He blamed the club's defeat by FBK Kaunas in the qualifying stages of the Champions League for the dramatic decrease in income.
In a statement released to the stock exchange he said: "The early elimination from European competition together with the reduced number of games resulted in turnover decreasing to £39.7m from the record high of £64.5m achieved in 2007/08.
"In total 49 games were played in all competitions in 2008/09 as against 68 in the prior year.
"This had a detrimental impact on ticketing and hospitality income, whilst the commercial area of the business suffered due to the lack of income from Uefa for European participation."
The club's new chairman promised to deliver a personal address on the current financial status at Rangers' annual general meeting on 7 December.
Mr Johnston said: "I give an assurance to all Rangers supporters that I will do everything I can to ensure that success is delivered both on and off the pitch.
"In terms of financial performance, the club has operated in the most challenging economic climate for many years."
He added: "Net operating expenses decreased by £8.6m to £48.2m, reflecting the reduced costs of non-progression in European competition, including player incentives, in comparison to last year.
"Clearly, costs at this level cannot be sustained without Champions League income."
Ally McCoist wants to take over Rangers when Walter Smith bows out
Manager Walter Smith has not bought a new player in almost 18 months and was told last January to trim back his squad.
The Ibrox club are actively seeking a new owner and have been placed on the official Takeover Panel list.
Murray, who relinquished his chairman's role in August, owns the bulk of shares through Murray International Holdings.
Rangers have faced pressure from Lloyds Banking Group to bring their costs under control, but the bank has been quick to dismiss suggestions that it was running the club's financial affairs.
Meanwhile, the Glasgow side are also waiting to find out the extent of their punishment for crowd trouble at the Champions League clash with Unirea Urziceni.
The club face a penalty after visiting fans clashed with stewards in Bucharest on 4 November.