A former director of the Financial Services Authority has condemned his successors' report on the collapse of RBS, criticising the current FSA's focus on "technicalities" rather than the "reckless" management culture instilled by ex-chief executive Sir Fred Goodwin.

Lindsay Thomas, who now runs risk and regulatory consultancy Sustainable Risks, said that while tomorrow's report, would primarily focus on the FSA's flawed oversight of the bank, it was likely to downplay the key internal reasons for the bank's collapse.

"The report will come as a disappointment to anyone with an interest in finding out more about the true reasons for RBS's collapse," said Thomas. "It will focus on technicalities rather than what was really happening amongst management inside the bank. It's going to focus more on what happened inside the FSA."

Thomas, who left the FSA in 2006 ascribed the report's approach to "Maxwellisation", a reference to a process whereby individuals criticised in a British inquiry must be given the chance to respond before publication. Principal players, including Goodwin and their lawyers, thus have the opportunity to review the content and ask for negative passages to be extracted.

"Much of the meat will have been taken out," said Thomas. "The downside of this is it causes the FSA to be less candid, and can lead to a rather problematic and time-consuming process. Invariably, the FSA will have erred on the side of taking things out."

He added: "By virtue of what's been taken out, the report will have the appearance of over-emphasising the role of the ABN Amro takeover. The report will not have any real value unless you're able to read between the lines, and can identify what's been taken out."

The 480-page report is expected to rule out new "enforcement action" against the bank's ex-directors. In May 2010, the FSA agreed not to take disciplinary action against RBS investment banking boss Johnny Cameron, conditional of his not working in a senior City role again. Cameron was not found guilty of any regulatory breach and admitted none.

According to reports, tomorrow's document will explicitly blame RBS's former board for "aggressive expansion", and for making a "reckless gamble" with its €71 billion takeover of Dutch bank ABN Amro in October 2007.

As has been publicly admitted the ABN deal was executed with limited due diligence, an admission that is understood to have given the regulator confidence to resist pressure from Goodwin's solicitors, Norton Rose, to have critical passages watered down or removed.

The report will also say the RBS board "tended to optimism", in its mid to late-2007 confidence that the credit crunch would be a temporary phenomenon, and that the US housing market would rapidly recover. Banking sources said the bank's former chairman Sir Tom McKillop, Goodwin and other directors have been "desperate" to water down the report since they are "running scared" of law suits from retail and institutional shareholders in RBS, who lost scores of billions as a result of its collapse.

Stewart Hamilton, emeritus professor of finance at IMD Business School in Lausanne, said: "I hope the report will address two key issues: first, the veracity of statements made by RBS in its April 2008 rights issue prospectus – and in particular the working capital statement. Secondly, I hope the FSA addresses the role of the non-executive directors and, in particular, the role of the audit committee, in the bank's governance."

The RBS audit committee was, at the time of its collapse, chaired by Archie Hunter, a former president of the Institute of Chartered Accountants of Scotland and ex-senior partner of accountancy giant KPMG in Scotland.

Hunter was an RBS non-executive director from 2004 until 2010. Other non-executives on RBS's audit committee at the time of its failure were former Treasury mandarin Sir Steve Robson and ex-JP Morgan investment banker Joe MacHale.

Publication may also trigger calls for FSA chief executive Hector Sants to step down, given that he was an executive director of the FSA during RBS's madcap quest for growth, at a time when the regulator did nothing to stop it, and became FSA chief executive in July 2007, 15 months prior to the bank's collapse.

Alan Steel, chairman of Alan Steel Asset Management, said: "There's no way Hector Sants should be in the position he is. If he was a director of a private-sector firm, he would have been struck off."