The Nursing and Midwifery Council has been rapped for the way it handles disciplinary cases - including Stafford Hospital complaints - in a report by the Professional Standards Authority.
The standard of investigations was poor, there was flawed decision-making and record-keeping was lax, says the PSA.
The NMC is responsible for regulating nurses and midwives throughout the UK. It has admitted "substantial failings" in the past - but claims the latest report recognises it is making progress.
The PSA did an audit of 100 cases dealt with by the NMC's disciplinary committee for its new report.
It found some improvements by a new management team at the NMC, but identified a number of weaknesses
The PSA report said: "We were disappointed that the NMC's internal review (in July 2013) of its handling of cases that involved registrants employed by Mid Staffordshire NHS Foundation Trust did not identify a number of serious issues that we picked up in our audit."
The report highlighted concerns over the use of voluntary removal, where a nurse or midwife facing a disciplinary hearing could apply to be removed from the register - without a full public hearing.
"We identified concerns in all 21 of the cases we audited that were closed following the grant of applications for voluntary removal, and have urged the NMC's Council to ensure that our concerns about the NMC's procedures are addressed quickly," the report said.
NMC chief executive Jackie Smith said there was "still more to do", but the said the picture was improving.
But Katherine Murphy, chief executive of the Patients Association, said the report was "deeply worrying".
"It is disgraceful to see that lessons are still not being learnt from the Francis Inquiry and serious issues relating to registrants from the Mid Staffordshire trust have not been picked up," she said.