The chair of the Mid Staffordshire Foundation Trust Public Inquiry has called for a rethink on minimum nurse staffing levels – less than six months after deciding not to recommend mandatory ratios in his landmark report.

Robert Francis QC last week suggested to regulators that a minimum staffing level should act as an “alarm bell” for questions about safety, in the same way as high mortality rates.

In February, nursing unions and other campaigners had been disappointed Mr Francis did not go further on staffing levels in his high profile report on Mid Staffs.

Although he called for nationally recognised tools for establishing appropriate staffing levels, he stopped short of backing the introduction of mandatory minimum nurse-to-patient ratios. The bulk of his recommendations for nursing instead focused on the culture and values of the profession.

However, during a public appearance last week, Mr Francis said he had seen evidence from the Safe Staffing Alliance – a confederation of nursing and patient groups set up earlier this year – that had convinced him the issue should be revisited.

Mr Francis stated: “It’s evidence… [that] ought to be considered with regard to whether there is some sort of  benchmark, which at least is a bit like a mortality rates – an alarm bell which should require at least questions to be asked about whether it is possible for a service to be safe.”

He stressed the level should not be viewed as “the adequate level of staffing, but the level below which you cannot be safe”.

The alliance – whose members include the Royal College of Nursing, Unison and the Patients Association – said in May that no ward should have more than eight patients to one registered nurse, plus a nurse in charge on acute surgical and medical wards.

Mr Francis was speaking last week at a board meeting of the Care Quality Commission, where the regulator’s response to his report was discussed.

His intervention comes after Sir Mike Richards, the CQC’s new chief inspector of hospitals, confirmed he was not planning to include staffing levels in the new surveillance model that will be used to decide which trusts to inspect – though they will looked at as causes of poor care such as unanswered call bells.

But Mr Francis urged the CQC to consider monitoring staffing levels as a “way to show real support for staff”.

Elaine Inglesby-Burke, director of nursing at Salford Royal Foundation Trust and a leading member of the Safe Staffing Alliance, said she hoped Mr Francis’ comments would influence the government’s final response to the Francis report, due later this year. “We are delighted that we have been heard,” she said.

Sally Brearley, chair of the prime minister’s Forum on Nursing and Care Quality, said a “pivotal moment in the debate on minimum nurse staffing levels” had now been reached.

Explaining his change of view, Mr Francis told the CQC that evidence heard by his inquiry in 2011 had been insufficient at the time to persuade him that a “minimum across the board staffing level” was needed.

“I was… only dealing with the events arising out of a particular hospital so the inquiry, for all its breadth, in the end had limitations,” he said.

An inquiry nursing seminar was told evidence on minimum staffing levels was inconclusive while in her evidence former chief nursing officer for England Dame Christine Beasley said her concern with mandatory ratios was “instead of becoming the floor they become the ceiling”.

Royal College of Nursing director of policy Howard Catton said nursing had been more divided over the issue when Mr Francis was hearing his evidence sessions than it was now.

“We have seen much more of a consensus across the profession that includes not just the ‘usual suspects’, like us, but leading nurse researchers, key nurse executive directors and the Patients Association as well,” he said.

He said Mr Francis’ apparent change of heart could help to “persuade others of the value and credibility of looking at nurse staffing levels as a warning sign”.

In a statement, Sir Mike said staffing levels would be considered during inspections.

He added: “Over time, we hope that our new inspection model will help to give a clearer picture of the link between staffing levels in hospital wards and the quality of care provided. This should contribute towards an evidence-base for making decisions on minimum staffing levels.”