An unprecedented array of key figures in the NHS will be castigated next week in the hard-hitting final report of a public inquiry into who caused – or failed to prevent – the Mid Staffordshire healthcare scandal. Between 400 and 1,200 people died unnecessarily as a result of treatment at Stafford hospital between January 2005 and March 2009.

The report by Robert Francis QC, who chaired an inquiry lasting more than two years, should help grieving relatives understand why poor care went unchecked for so long. But it will be devastating for the NHS, and a large number of managers, health professionals and regulators will be criticised for their role in the worst hospital scandal in recent memory.

They will be accused of incompetence, misjudgment and not responding properly to evidence that patients' safety was at risk from neglect, too few staff and sometimes inhuman care.

Francis's report was delayed in October after he sent warning letters to 20 individuals or organisations. According to sources close to the inquiry, the letters set out privately and in advance the criticisms he intended to make publicly in his final report and the evidence on which he had based his conclusions.

Many recipients, who were among the 164 witnesses who gave evidence during 139 days of hearings over 37 weeks between November 2010 and December 2011, sought legal advice before replying.

"Nobody in the NHS will emerge from Francis with any credit, I think – from the most senior NHS leaders to everyone at the hospital itself," said Katherine Murphy, chief executive of the Patients Association, who had core participant status at the inquiry.

"I hope the report is wide-ranging, in-depth and unsparing in explaining why these things happened and why NHS regulation failed so abysmally and why senior NHS figures behaved as they did."

Francis was asked in June 2010 by the then health secretary Andrew Lansley to conduct the inquiry, after he had recommended in an earlier report into standards of care at the hospital that there should be "independent scrutiny of the actions and inactions of the various organisations to search for an explanation of why the appalling standards of care were not picked up".

Francis's brief was "to examine the operation of the commissioning, supervisory and regulatory organisations and other agencies, including the culture and systems of those organisations in relation to their monitoring role at Mid Staffordshire NHS Foundation Trust between January 2005 and March 2009, and to examine why problems at the trust were not identified sooner and appropriate action taken".

His report will also "identify the lessons to be drawn from that examination as to how in the future the NHS and the bodies which that regulate it can ensure that failing and potentially failing hospitals or their services are identified as soon as is practicable".

His dissection of who did and did not take appropriate action is widely expected to examine the role of people and bodies including:

• The hospital trust's chairman, Toni Brisby, and board of directors.

Francis's first report in 2010 said that "the culture of the trust was not conducive to providing good care for patients" and identified problems including "bullying; target-driven priorities; low staff morale; lack of openness; acceptance of poor standards of conduct; [and] denial".

The board conducted more of its business in private than was appropriate and preferred denial of problems to the searching self-criticism needed, he said. Board members should have asked more questions about operational issues at the hospital and not restricted their role to helping to provide strategic direction.

The board was also heavily criticised in 2009 in a report by the Healthcare Commission (HCC), which lost faith in its ability to properly examine complaints about care. Brisby became the trust's chairman in 2004, but she stood down at the request of Monitor, which regulates foundation trust hospitals, just before the HCC's criticisms were published. She later said the HCC had used "shaky evidence" to "vilify" the hospital.

• Hospital trust chief executive Martin Yeates. He maintained that he had turned around a failing hospital trust and improved its quality and standard of care. But Francis's first report was scathing about Yeates's belief that the hospital's high death rates were due to issues of classification rather than poor care, and doing too little to tackle staffing problems. "He considered the HCC report to be unfair. Whatever Mr Yeates may have believed at the time of his departure, in reality the issues raised in this report had not been remedied", said Francis.

Yeates reportedly walked away with a £400,000 payoff, but the trust said he had got half his £170,000 salary. He did not appear as a witness at either Francis inquiry, citing ill health. He had lost his "family, career and health" and the affair had made his "life hell, a genuine living nightmare", he said in a statement. He and Brisby left the trust in March 2009.

• The NHS strategic health authority covering the West Midlands. Francis is expected to criticise the SHA, the regional arm of the Department of Health, for not exercising enough oversight of Mid Staffs or inquiring properly into mounting concern from patients and relatives from late 2007. Critics say it was too close to the trust and too readily accepted the trust's insistence that apparently high death rates revealed by hospital standardised mortality ratios (HSMR) data were due to classification issues rather than poor care.

Under Cynthia Bower, its chief executive until 2008, the SHA commissioned a report into the HSMR figures from academics known to be sceptical about their value. The SHA also sought to have the HCC's damning report into the emerging scandal at Stafford curtailed. Peter Shanahan, Bower's successor in August 2008, also tried to have the HCC's report toned down and challenged its estimate of 400-1,200 deaths.

• The Healthcare Commission. It was the NHS regulator in England until April 2009 and was the first body to reveal the extent of the "appalling" care at Mid Staffs through its report in March 2009, which Francis later praised. Critics, though, including the Commons health select committee, said it should have known more and acted sooner. But the then-HCC chairman, Sir Ian Kennedy, insisted it had uncovered problems others had missed.

"Everyone bought the [trust's] story, except us. The degree of risk to patients was not clear, but we were suspicious. Our surveillance techniques got under the skin of the story. We went in. We exposed what was happening," he said.

• The Care Quality Commission (CQC). It replaced the HCC in April 2009 and Bower became its inaugural chief executive after leaving the SHA. Critics say that regulation of hospital standards under her leadership was too light-touch, and she later agreed to make it more hands-on. The watchdog was mired in constant controversy during her time, with health organisations, the select committee and even the body representing hospitals raising concerns that its monitoring of NHS providers was inadequate.

On its last day the public inquiry heard unexpected evidence about alleged CQC failings from board member Kay Sheldon and investigator Amanda Pollard. "The organisation is badly led with no clear strategy," and staff were afraid to raise concerns, said Sheldon. The CQC's weaknesses meant it would not necessarily "spot another Mid Staffordshire", said Pollard. As health secretary from 2010-12, Andrew Lansley never trusted Bower's ability to run the CQC. Bower resigned last February, as did the CQC chair, Jo Williams, late last year.

• NHS chief executive, Sir David Nicholson. Tom Kark, counsel to the public inquiry, said Nicholson's belief that the failings at Stafford were "singular" and did not indicate a wider systemic problem in the NHS was a "dangerous attitude", and that he had been "naive" to expect the NHS's regulatory system to detect the poor care. Julie Bailey, leader of the patients' group Cure The NHS, believes he should lose his job because of that and for not asking more questions about events at the trust and intervening. Nicholson, though, recently said "nothing I've seen" about his role in Mid Staffs made him think his position was under threat.

• The Department of Health, including both health secretaries from the time the Mid Staffs scandal was unfolding – Alan Johnson and Andy Burnham both dealt with it – and civil servants, including then permanent secretary Hugh Taylor, and others, including those who helped Mid Staffs become a foundation trust hospital, semi-independent of the department. Relatives say complaints and requests for meetings were ignored.