A group of MPs have joined a call by Nicky Morgan, the Treasury Committee chair, for full publication of a leaked report on the treatment of customers in RBS's global restructuring group (GRG).
The report, produced for the Financial Conduct Authority (FCA), suggested that GRG mistreated many of its clients.
RBS denies the claim.
The All Party Parliamentary Group on Fair Business Banking called RBS's treatment of business customers an "ugly stain" on the industry.
Earlier, Mrs Morgan asked FCA head Andrew Bailey to secure RBS's permission to publish the report "without delay".
She says the report is in the hands of an "unknown number of third parties".
RBS's global restructuring group operated from 2005 to 2013 and at its peak handled 16,000 companies.
It was introduced as an expert service that would turn around a business and stepped in when companies missed a loan repayment or had a drop in sales or profits.
But the report, seen by the BBC, found that struggling companies who were placed in the recovery group had a slim chance of emerging from it.
The letter from the All Party Parliamentary Group on Fair Business Banking expresses a number of concerns.
Among these is that the investigation may not have included all customers involved with the GRG, since RBS was left to decide which businesses were to be looked at.
It says in its letter that while details of the "focus and execution" of the investigation remain hidden from public view, there cannot be confidence that the probe has identified the scale and damage of the poor behaviour at RBS, nor whether there has been an adequate response from either the bank of the FCA.
The head of the group, Lord Cromwell, is meeting Mr Bailey next week.
Mrs Morgan said: "The FCA told the committee in November 2016 that a 'full account' of the findings from the skilled persons' report would be published.
"Nearly a year later, and nearly four years since the report was commissioned, we are still waiting for answers."
"I have asked Mr Bailey to update the committee on any information that the FCA uncovers as part of its inquiry into the leak," she said.
"This would not be the first instance of leaking from the FCA, but lessons must be learned to ensure it is the last."
The FCA said it would respond "in due course" to the request from Mrs Morgan.
"We have already initiated a leak inquiry into the disclosure of the s166 report on RBS GRG to the BBC, and we have asked the other parties who had access to the report, namely RBS and Promontory, to do the same.
"If the Treasury Select Committee or the BBC have evidence that the document was leaked by the FCA, we encourage them to share that with us."
Bill Esterson, Labour's shadow business minister, said Mrs Morgan should not be asking RBS for permission to release the report, "she should be demanding it... What happens if they say no".
He said: "Livelihoods were ruined... Of course the report should be released and a full enquiry held."
In November 2013, Lawrence Tomlinson, then 'Enterprise Czar' for Business Secretary Vince Cable, made several allegations against RBS in a report into the GRG.
On the same day, RBS chairman Sir Andrew Large published an RBS-commissioned report into its own lending performance, which said that the bank needed "to address the concerns that have been raised by some customers and external shareholders".
Two months later the FCA announced its own review into the group's conduct.