The Financial Conduct Authority (FCA) has attempted to draw a line under a five-year scandal that saw officials accused of failing to take action against poor practices at Royal Bank of Scotland's controversial restructuring division.

The Global Restructuring Group (GRG), which was part of RBS, was supposed to help small businesses after the financial crisis in 2008, but instead ruined thousands of livelihoods through allegedly asset-stripping to shore up the bank's balance sheet.

Previously, the FCA has said it was unable to investigate the GRG because its work was outside the regulator's remit. But on Thursday a final report into the scandal said new rules mean that, in future, similar situations would fall under its regulation.

However, the 78-page report failed to say whether the FCA would have been able to bring a successful case against the GRG managers, even with the new powers.

It said: "We cannot say whether we would have been able to bring successful cases against RBS senior management had the (new rules) been in force during the review period.

"This would involve applying a new regime to a historic set of facts and it would not be appropriate for us to make a hypothetical judgment."

Attempts to end the saga were met with derision, with the All-Party Parliamentary Group on Fair Business Banking (APPG) calling it a "whitewash".

Kevin Hollinrake MP, co-chairman of the APPG, said: "This report is another complete whitewash and another demonstrable failure of the regulator to perform its role."

He added that the report was supposed to consider the "root causes" of the problems and to see if senior management at RBS knew about the controversy.

"They have manifestly failed to do this."

The Federation of Small Businesses (FSB) also attacked the report, saying it "failed to provide consolation to former GRG customers who lost everything".

National chairman Mike Cherry said: "It's troubling that this report does not provide assurance that the Senior Managers & Certification Regime would be effective were a similar scenario to arise again."

FCA chief executive Andrew Bailey, who is also in contention for the Bank of England Governor job, said: "GRG has been highly damaging for those customers impacted and more widely for the reputation of the banking industry.

"Combined with other issues that have impacted SMEs, it is important for all who work in this sector to regain the public's trust.

"I must acknowledge the distress felt by many of GRG's customers.

"The firm's relations with its customers were often insensitive, dismissive and sometimes too aggressive; these failings made an already stressful situation worse.

"I know that many customers of GRG therefore disagree with our decision to not take enforcement action, but I hope that this report will explain why we reached that decision."

Despite the strong words against the GRG's behaviour, the report also said "the evidence does not suggest that management sought to treat customers unfairly".

One option for the FCA to punish managers at the GRG could have been to declare them no longer "fit and proper" but officials said this label could not apply.

It said: "We found no evidence that any member of senior management was dishonest or lacking in integrity. In particular, we have not found a credible basis to conclude that senior management sought to treat customers unfairly or behaved in any other way that could call their honesty or integrity into question."

The report also refused to name any of the senior managers involved in the scandal, citing legal reasons.

Thursday's report was issued after the Treasury Select Committee demanded details of why the FCA could not investigate and censure the GRG. A summary was initially published, but MPs demanded the full report.

The new rules will see senior managers at financial institutions fall under the gaze of the FCA for future investigations, even if the work they carry out is not regulated by the watchdog.

Sir Howard Davies, chairman of RBS, welcomed the FCA final report.

He said: "The bank has acknowledged that some SME customers did not receive the treatment they should have done while in GRG during the relevant period and has apologised.

"The way the bank deals with business customers in financial difficulty today is fundamentally different to the aftermath of the financial crisis, during what was a hugely challenging time for the bank, its customers and the wider economy."