Patients are being let down by the Nursing and Midwifery Council (NMC) because of "failings at every level", a report has found.
A root and branch overhaul is required for the NMC to fulfil its key duty to protect the public, the review by the Council for Healthcare Regulatory Excellence (CHRE) found.
The NMC is responsible for the regulation of the 670,000 nurses and midwives currently registered in the UK.
But a succession of weak leadership has failed to created the modern, effective and efficient regulator that the public, nurses and midwives need and deserve, the review found.
Despite a "strong passion for public protection" among staff, the report found problems "at every level, in every system".
The report revealed that the number of cases being dealt with by the NMC had risen by almost 50% since 2009, to more than 4,000 cases.
But delays in investigating complaints caused a backlog, and 1,500 are yet to be heard.
About a third of cases carry forward into the next year, and some cases take up to five years to close.
Bad morale has led to resignation and acceptance of poor standards among staff and almost a third of them leaving within the last year.
The report outlines 15 recommendations in which it says new leadership is vital to "provide and sustain clear direction" to ensure public protection and public confidence in the NMC, and to challenge its culture of "resigned resilience".
Jackie Smith, the NMC's acting chief executive and registrar, apologised for what she called its "substantial failings".
She said: "The strategic review report and annual performance review report together make difficult reading for the NMC.
"They highlight substantial failings in the delivery of our regulatory functions and in the management of our organisation.
"We recognise the failings that CHRE have set out in their reports, and we are sorry. It is clear that the NMC has not delivered effective and efficient regulation, and we are committed to putting that right.
"We look forward to the appointment of the chair and chief executive later this summer. Change is already under way and will continue, to ensure the NMC's focus is wholly on public protection."
Louise Silverton, deputy general secretary of the Royal College of Midwifery, said the report should be "a wake-up call".
She said: "We agree that the NMC must focus on its core regulatory functions and responsibilities, notably protecting the public and ensuring it maintains its confidence and that of the profession it regulates.
"We are pleased that the CHRE recognises that the NMC must acknowledge the difference in the way it regulates the two professions of nurses and midwives.
"The RCM looks forward to working with the NMC through this process and through our work on developing advice, standards and guidance for midwives. We need to find a way forward that embraces change."
The union Unison, which represents nurses and midwives, welcomed the report but criticised an increase in its members' registration fees to £120 to "help pay for its mistakes".
Gail Adams, Unison head of nursing, said: "It is perverse to demand that registrants should bail out organisations who are in trouble as a result of their own financial mismanagement - particularly nurses and midwives who have had to endure a two-year pay freeze in increasingly difficult financial times.
"We fail to see why nurses and midwives should have to bail the NMC out of a financial crisis of its own creation."
Public Health Minister Anne Milton said: "Strong regulation of nurses and midwives is crucial to ensure public protection, and effective leadership within the Nursing and Midwifery Council is key to this.
"We would like to thank the Council for Healthcare Regulatory Excellence for its work and we expect to see the NMC implementing all of the recommendations contained within this report.
"In light of these recommendations I have requested urgent assurance about the process for the appointment of the NMC chair."