Staffing levels are so low in parts of crisis-hit Stafford Hospital the loss of one more worker could make services unsafe, a top health inspector has warned.
The Chief Inspector of Hospitals professor Sir Mike Richards said he was "surprised and very concerned" no plan had yet been drafted ensuring "safe transition of responsibility for clinical services" at Stafford, and urged it be done without delay.
Welcoming Sir Mike's comments, the hospital's bosses said they were working to "finalise detailed planning" ahead of a long-planned transfer of services to neighbouring health trusts in November.
The chief inspector's conclusions follow a three-day safety inspection carried out by the Care Quality Commission (CQC) over June and July at the invitation of the hospital's current bosses.
Mid-Staffordshire NHS Foundation Trust, which currently runs Stafford, is scheduled to be dissolved and its services taken over by neighbouring hospitals later this year.
The transfer is being handled by the trust special administrators (TSAs).
In 2013, the independent Francis Report into the Mid-Staffs trust concluded there had been basic failings in standards of care at Stafford, with hundreds more patients dying than would have been expected between 2005 and 2008.
TSAs were later appointed to run Mid-Staffs after foundation trust watchdog Monitor determined the trust was no longer clinically or financially viable in the long-term.
But since then Mid-Staffs has struggled to recruit and keep staff in part because of the fall-out from the crisis.
Sir Mike said the results of the urgent inspection concluded services were safe, but staffing levels were only just adequate in some areas, and particularly on medical wards.
The inspection team concluded it was "not assured about the sustainability of services", warning that "should recruitment or retention fall by even one or two people in some key posts, services would become unsafe".
Sir Mike added the problem of finding staff to cover shifts was taking up the time of senior managers, who are working on the transfer of services from Stafford and its sister hospital Cannock Chase.
The chief inspector said: "The senior managers at MSFT (Mid-Staffs), including the chief executive, are having to spend inordinate amounts of time ensuring that individual nursing shifts are adequately filled and that sufficient numbers of medical staff will be available for different services.
"To date they have been able to do this, but I would emphasise the word just.
Sir Mike added: "We were both surprised and very concerned a clear transition plan has yet to be developed to ensure the safe transition of responsibility for clinical services to the agreed model of care over the next four months.
"This clearly requires the full involvement of MSFT and other organisations in the wider health economy."
He said staff at Stafford Hospital needed clarity "as soon as possible about what is going to happen next" and said the current uncertainty was "contributing to fatigue and fragility among staff".
Calling for the transition plan to be drafted as soon as possible Sir Mike said it "should include a commitment by the acquiring organisations to support medical and nursing staff levels at Mid Staffs over the next four months so that services remain safe".
He has written with his findings to Monitor, the NHS trust development authority, the TSAs and also University Hospitals of North Staffordshire Trust and Royal Wolverhampton Hospital Trust, which will be taking over the services.
Last month, the TSA said it had invited inspectors in to Stafford because the "fragility of services due to staff recruitment and retention" had become "a significant challenge".
In a joint statement, Monitor, the TSAs and the trust development agency said: "We welcome Professor Sir Mike Richards' feedback which recognises the considerable efforts by staff and the Trust Special Administrators to maintain safe services at Stafford Hospital in the short term.
"Patients at the hospital deserve to have long term access to safe, good quality services and this is what the TSAs' recommendations aim to secure. In light of this report, the local health economy will be working together to finalise the detailed planning and begin the transfer of services as planned."
A full CQC report is expected to be published in August.