Royal Bank of Scotland executives "misled" MPs over the bank's treatment of small companies, Labour's Clive Lewis has told the House of Commons.
RBS bosses appeared before the Treasury Select Committee and said cases of mistreatment had been "isolated".
But Mr Lewis said he had a full copy of the Financial Conduct Authority's (FCA) report, which, he said, stated the bank's behaviour had been "systemic".
RBS said it would "strongly deny" that it misled MPs.
"The evidence we provided to the Treasury committee accurately reflected the bank's position.
"We are not clear on what basis the allegations are being made, but we would strongly deny the suggestion that we misled the committee," it said in a statement.
The FCA report, which has not been made public in full, examined the conduct of the RBS's Global Restructuring Group (GRG), which dealt with more than 12,000 companies between 2007 and 2012.
The report, details of which were first published by the BBC, found that 92% of viable companies had received "inappropriate" treatment".
Mr Lewis, who is Labour's Treasury spokesman, said: "Having read the document, I believe it shows that RBS executives misled the Treasury Select Committee in their evidence and had a stated policy of misleading members of this House."
Speaking in the House of Commons last month, Mr Lewis described RBS's treatment of small businesses as perhaps the "largest theft anywhere, ever".
On Tuesday Mr Lewis told MPs: "Since that debate, I have received a full and unredacted copy of the Financial Conduct Authority's investigation into RBS, which the FCA has so far refused to release, including to the Treasury Select Committee.
"Far from being isolated incidents of poor governance, as they claimed to the committee, this report explicitly states their behaviour was 'systemic and widespread'."
He said: "In one shocking passage of the report, out of hundreds, the bank boasted one family business was set to 'lose their shirts' so RBS could get a 'chunky equity deal' - meaning the bank would acquire its assets cheaply.
"Furthermore, it is clear the summary of the report the FCA has published is what I would politely describe as a sanitised version," added Mr Lewis.
Mr Lewis asked for permission to hand the unredacted report to Nicky Morgan, who chairs the Treasury Select Committee, and Commons Speaker John Bercow.
Ms Morgan said the FCA had consented to publish the report once it had completed its investigations into conduct at GRG.
"The committee expects these investigations to be completed swiftly and will ask the FCA to confirm its intention to do this at its evidence session tomorrow [Wednesday].
"The committee has the power to require the FCA to produce the report.
"The committee's priority has always been to bring maximum transparency to events at GRG.
"In doing so, the committee must ensure that the information it receives is accurate and complete.
"For this reason, were it to decide to publish the report, it would generally look to the FCA, as the body that commissioned the GRG review, to provide it."
Analysis: By Joe Lynam, business correspondent
A week after the RBS chief executive appeared before the Treasury select committee and a day before the FCA chief executive is set to appear before it, this dramatic intervention by Clive Lewis will up the ante even further on the bank and its regulator.
Being protected by parliamentary privilege, allowed the shadow Treasury minister to accuse the bank of misleading MPs. But more than that, having read the report he said that RBS board was aware of GRG's activities.
And Mr Lewis didn't hold back on the FCA's role either. His accusation that it published only a 'sanitised version' of the full report will be cheered by GRG victims but jeered in Canary Wharf.
The last thing the FCA will need is the impression that it backed the bank above thousands of small firms.
And the Government is also in the firing line. Another Labour MP, Bill Esterson, was critical today of the Government's silence on GRG and its treatment of small and medium sized enterprises.
In response the Treasury said the investigation into GRG was a matter for the independent Financial Conduct Authority.
An FCA official said the summary of the report had been reviewed independently by Andrew Green QC, who was appointed by the Treasury committee and had "concluded that it was a fair and balanced account of the findings of the report and closely reflected its tone and narrative".
A spokesman for the GRG Business Action Group, which represents more than 500 businesses, said: "What Clive Lewis said in Parliament today is extremely worrying and surely makes immediate publication of the FCA report an urgent priority in the public interest.
"He described the FCA's published summary as a 'sanitised version' of the actual report.
"In view of the FCA's repeated assurances that it is not favouring RBS, this is a shocking allegation.
"[FCA boss] Andrew Bailey will have to explain himself before the Treasury Select Committee tomorrow."