A “sweet and affectionate” six-month-old boy died when doctors misdiagnosed his heart complaint as pneumonia, a highly critical report by the NHS ombudsman has revealed.

Harris James died in 2015 after a catalogue of errors by staff at James Paget hospital in Great Yarmouth, Norfolk. The hospital then compounded his parents’ anguish by mishandling his mother’s complaint and failing to apologise for its failings.

If he had received proper care he would have survived, according to the results of the ombudsman’s inquiry.

Harris was referred to the hospital on 2 November by his GP, who was concerned that he had lost weight after suffering a bout of gastroenteritis. The GP recorded in the referral note that an area of the boy’s stomach just below his ribs was drawn inwards. However, staff did not include that in his medical records. They performed blood and urine tests on him and arranged for him to see a dietitian four weeks later.

But on 12 November Harris was rushed back to the hospital because he had vomited and become limp. A chest X-ray showed that part of his lung had filled with fluid and an electrocardiogram disclosed that he had several abnormalities in his heart.

Crucially, however, staff at James Paget wrongly suspected that Harris had pneumonia and gave him oxygen, antibiotics and fluids. They did not ask a cardiologist about his abnormalities.

He did not see a heart specialist until the next morning. Soon after he had a heart attack and died.

“Our son was an affectionate and sweet little boy whose sudden death devastated our family”, said his parents Mary Gunns and Ryan James. “We won’t ever be able to forgive James Paget hospital for its failings, nor will we forget the additional pain caused by its mishandling of our complaint.”

They hope that the changes the James Paget NHS trust has pledged to make in an action plan as a result of the ombudsman’s inquiries means that “this never happens again”, they added.

Rob Behrens, the parliamentary and health service ombudsman, found that in their care and treatment of Harris staff had failed to:

Act on the results of the ECG and chest X-ray.

Consider Harris’s medical history and symptoms.

Ask specialist staff for their input.

Escalate his care when his condition worsened.

“If these failings had not occurred it is likely that the trust would have recognised that [Harris] had a problem with his heart. In these circumstances he would have received the correct treatment instead of being treated for suspected pneumonia.

“We found that on the balance of probabilities his cardiac arrest would not have occurred, and it is more likely than not that his death would have been avoided”, added the ombudsman’s report.

Anna Hills, the trust’s chief executive, said it had apologised to Harris’s family both for the clinical mistakes made and also for “the manner in which we communicated with them and handled their complaint when they raised concerns after his death”.

The trust will also pay Gunns, a local government officer, £15,000 “in recognition of the injustice its failings have caused”, Behrens added.

“This is a very tragic case but what makes it worse is that the clinical failings were compounded by a poor investigation and response to the complaint, adding insult to injury.

“Ideally, the trust should have recognised its failings, apologised to the family and made sure it didn’t happen to someone else”, said Peter Walsh, chief executive of the patient safety charity Action against Medical Accidents (AvMA).

“Sadly failures to diagnose and treat appropriately also happen too often. The reasons can be complex but short staffing is often a contributing factor”, he added.

• This article was amended on 18 and 19 December 2019 to correct the first names of the parents of Harris James.