It is believed the Glasgow club is being investigated over offshore payments made to players over a period of ten years.
Football finance experts yesterday insisted that the potential extra tax bill could make the sale of the club, put on the market by owner Sir David Murray after he stepped down as chairman last year, more difficult.
But Rangers last night insisted they would defend themselves "robustly" against the claims. A club spokesman said: "There is an ongoing query raised by HMRC, which is part of a pending court case.
"On the basis of expert tax advice provided to Rangers, the club is robustly defending the matters raised. It would, therefore, be inappropriate to comment further at this stage.
"There is endless speculation about the future of the club, much of it ill-informed. It is to no-one's benefit to comment on every speculative opinion."
The offshore payments are believed to have been made over the last decade to some of the club's biggest stars. It is understood the money goes into an "employee remuneration trust" held in a tax haven overseas.
This is a controversial loophole that has seen the taxman launch investigations into a string of English Premiership clubs who have also used the same payment methods in players' contracts, along with tax-free payments for image rights.
HMRC has been aggressively pursuing clubs such as West Ham and Portsmouth over similar payment methods.
The Ibrox club, who won the Scottish Premier League on Sunday, are 31 million in debt and have faced strict spending cuts imposed by Lloyds bank. Their last major signing was American midfielder Maurice Edu, whom they picked up for 2.6m from Toronto in August 2008.
Tax officials yesterday refused to confirm any details of their investigation into Rangers. An HMRC spokeswoman said: "HMRC cannot comment on the confidential tax affairs of individuals or businesses."
The case could see the club hit with a backdated tax bill running to millions, which experts believe could act as a barrier to the future sale of the club.
"The bill for this, when HMRC complete its investigations, could be double figures in the millions. No-one knows the exact figure, but who is going to buy the club unless someone agrees to pick up that bill?" said one football finance expert.
"It is the ultimate poison pill and the investigation could be some time from its conclusion."
It has been suggested that London-based property developer Andrew Ellis is likely to launch a takeover bid for the club. Director Dave King, in a meeting with fans in Johannesburg last week, also said he wanted to buy the club. It has been suggested he was prepared to offer 18m to Lloyds and 1 to Sir David Murray for his shareholding.
The vast majority of the 15m windfall brought in from next term's Uefa Champions League ties is expected to be used to reduce debt as Rangers are forced to work under a crippling business plan imposed by Lloyds.