The Advertising Standards Authority has insisted it has the skills to assess adverts about tax and financial planning.
The watchdog's comments come after the ASA upheld complaints by HM Revenue & Customs about a financial advice firm offering a contractor loan scheme.
But Mark Turner, director at Williams Gordon, said the decision was a "whitewash".
He said: "The ruling doesn't represent the arguments we put forward, so it is very difficult for us to see how they upheld it."
He added that the 92 per cent claim was "feasible depending on people's rates.
Mr Turner said: "They upheld every single argument no matter what we said."
George Bull, a senior tax partner at RSM, said he had now seen a couple of cases where the ASA had forced promoters to withdraw misleading claims after HMRC's intervention.
He said: "While I have no problem with either of those examples I do have some concerns about the governance here.
"The ASA's published decision contained a fair amount of tax analysis - the latest one goes into the tax avoidance disclosure rules in some detail – which begs the question is the ASA equipped to carry out this sort of review?
"This is an unusual development and something which will need to be monitored in the future."
But a spokesman for the ASA said it was not responsible for addressing tax avoidance, instead its focus is on misleading advertising.
He said: "The ASA is well equipped to assess if advertising claims about tax are correct.
"We have over 55 years of experience of conducting investigations, assessing evidence and administering the advertising codes.
"Our staff are trained to evaluate all manner of evidence; and we have staff with specialisms in particular areas, including finance. We also operate a reverse burden of proof. It is for an advertiser to prove its claims are not misleading rather than for us to disprove them.
"In judging ad claims around tax avoidance we will, where appropriate or required, seek the view of independent experts and the HMRC. Their insights help inform our decision making but, ultimately, any final decision on whether the advertising rules have been broken sits with us."
Earlier this week the ASA upheld a complaint against Williams Gordon, which offers solutions for contractors and had made a claim on its website that customers could "take home up to 92 per cent of your pay", and described the solution as "legally robust".
The ASA upheld three complaints against Williams Gordon, all made by HMRC.
These included whether the claim that you could take home 92 per cent of your pay was misleading, and the claim that "We are fully compliant with the necessary HMRC legislation and all current IR35 policies. We also ensure that you remain tax compliant for the duration of the contract" and "Not only providing the highest return, but doing so in a legal, robust, documented and defendable way" was inaccurate.
A HMRC spokesman said: "HMRC works with a number of partner agencies, including the ASA, to tackle marketed tax avoidance, and won't hesitate to challenge misleading claims by promoters of schemes."
damian.fantato@ft.com
