Rangers fans groups have objected to the appointment of Harper MacLeod to investigate claims against the club's previous owners.
The Glasgow law firm were instructed by the Scottish Premier League to probe allegations of dual contracts.
But a joint statement from three fans groups has questioned the process that could lead to the stripping of titles.
"Harper MacLeod have acted for Celtic FC, who would be the main beneficiaries of any finding of guilt," they state.
"Indeed, until it was recently removed, they included a testimonial from Celtic CEO Peter Lawwell on their website.
"The SPL could have chosen any law firm to carry out this investigation and it was extremely naive of them to appoint a firm with such a close connection to a rival club."
The Rangers Supporters Assembly, the Rangers Supporters Trust and the Rangers Supporters Association suggest that, in discussing the stripping of titles, "it appears the SPL has assumed guilt when nothing has yet been proved".
They urge the football authorities to ensure that the investigation is carried out "with the necessary independence, transparency and integrity".
Rangers' use of employee benefits trusts to pay members of staff remains the subject of a tax tribunal.
- The SPL is investigating how Rangers paid players between 2000 and 2011
- SPL said in June Rangers had "a prima facie case to answer in respect of [league] rules"
- Independent commission will rule on matter
- There are 18 available sanctions should the club be found in breach of SPL rules
While the courts have yet to rule on their legality, the SPL is investigating whether such earnings contravened its requirement that all earnings should be declared to the national parent body.
The SPL has stated that, once Harper MacLeod submits its evidence,
an independent commission,
likely to be made up of foreign nationals, will determine what sanctions, if any, are appropriate.
However, Rangers' fans groups state: "We are concerned that, despite the SPL having 18 sanctions available, the only punishment being discussed is the most serious, yet no guilt has even been established.
"This, again, leads to a danger of exerting undue influence on the tribunal members through the media.
"We believe this is being driven by influential members of the SPL board and recently formed 'steering committee' with the most to gain."
A BBC documentary, "Rangers, The Men Who Sold The Jerseys", found evidence suggesting that 38 Rangers players had received side-letters giving undertakings to fund their sub-trusts with cash.
The fans groups suggest that such media coverage, and similar newspaper articles, will make it difficult for any investigators to remain impartial.
Celtic have previously confirmed that they established one EBT scheme in April 2005, which BBC Scotland understands was for the benefit of Brazilian midfielder Juninho Paulista.
"We would question why there appears to be no on-going investigation into the so-called 'Juninho EBT' used by Celtic FC," add the Rangers fans groups.
"Our understanding is that Celtic acknowledged that this was not declared to the football authorities as part of their licence application."
The SPL did not wish to comment and Harper MacLeod have yet to comment on the supporters' statement.