An Health and Safety Executive (HSE) spokesman said:
Following legal advice, HSE deferred a decision to pursue the investigation into Gillian Astbury's death until the conclusion of the public inquiry, chaired by Robert Francis QC, into Mid Staffordshire NHS Foundation Trust.
We can now confirm that our inspectors have today formally started an investigation.
Our focus will be on establishing whether there is evidence of the employer (the Trust) or individuals failing to comply with their responsibilities under the Health and Safety at Work Act.
The report by Mr Francis highlighted "appalling and unnecessary suffering of hundreds of people" at the Trust between 2005 and 2009.
As many as 1,200 patients may have died needlessly after they were "routinely neglected" at the hospital.
Many were left lying in their own urine and excrement for days, forced to drink water from vases or given the wrong medication.