The ruling by the Scottish Public Services Ombudsman, Jim Martin, comes just weeks after the Government announced a nationwide roll-out of similar guidance on the way A&E departments deal with patients who do not require emergency treatment.
Mr Martin found "significant failings" after NHS Tayside repeatedly failed to diagnose cancer in a 32-year-old woman with excruciating back pain, even though an MRI scan showed a conspicuous abnormality.
On one occasion the patient, who was left barely able to walk due to the pain she was in, arrived at A&E at Ninewells Hospital in Dundee but was referred back to her GP under the health board's rules as her symptoms had persisted for more than three days and she had already consulted her family doctor. It was another month before cancer was diagnosed.
Earlier this month the Scottish Government announced that guidance based on the NHS Tayside A&E policy in operation at Ninewells, which it said had helped to ensure waiting times targets were met, would be rolled out throughout the country and could mean more patients being redirected to GPs from emergency departments to ease pressure.
Frances Reid, Director of Public Affairs and Services at Target Ovarian Cancer, said the case highlighted by the ombudsman underlined fears that other patients could become victims of misdiagnosis as a result of the policy being adopted nationwide.
"Around one-third of women receive their ovarian cancer diagnosis following an admission to an A&E department," she said.
"Typically they are either attending because they are in acute pain or have extreme abdominal distension, or because they have made repeated visits to their GP without the correct diagnosis being made.
"We do have concerns that the A&E guidance policy being rolled out across Scotland has the potential to further delay the diagnosis of these women with devastating consequences."
Mr Martin recommended that the health board reviewed the application of its "three-day guidance". His expert adviser said the policy was a "reasonable one if appropriately applied" but could be unsafe if taken in isolation.
The patient, who received an MRI scan on October 10, 2011 and visited A&E 16 days later, was diagnosed with cancer in late November that year. She died on April 1, 2012. While it may not have altered the patient's prognosis if a correct diagnosis was made sooner, more effective treatment could have been in place.
NHS Tayside has apologised to the patient's family and accepted the ombudsman's seven recommendations. The health board said no A&E patient was redirected without the assessment of a senior clinician.
A spokesman for the Scottish Government said: "Our new A&E guidance is designed to make sure all patients are getting the right treatment, in the right place. However, we have been very clear all patients should be assessed by a senior clinician before a decision is taken on the most appropriate treatment for them."