Dozens of patients have experienced delays from a new NHS system which "failed" to schedule a three-year-old boy for a vital hospital scan, his parents said today.
Samuel Starr was born with a complex congenital cardiac defect but made a good recovery following surgery shortly after his birth in 2010.
However, medics said Samuel, of Frome in Somerset, would still need regular tests to check on his progress.
But a delay came after a new computer system, Cerner Millennium, was rolled out at at the Royal United Hospital (RUH) in Bath, where Samuel had check-ups.
His parents and nurse repeatedly chased the hospital but a "glitch" meant Samuel was left without a crucial scan for 20 months after his first major operation.
By the time he was finally seen, Samuel's condition had dramatically deteriorated and he required urgent heart surgery at Bristol Royal Hospital for Children.
Samuel suffered a stroke during the procedure and died a month later, in September 2012, in the arms of his devastated parents, Catherine Holley and Paul Starr.
Avon Coroner Maria Voisin, following a three-day inquest in Flax Bourton, Bristol, ruled the "failure" of the booking system left Samuel without examination and treatment.
Speaking after the inquest, in which Ms Voisin reached a narrative conclusion, Samuel's parents claimed other lives were put at risk by the Cerner Millennium system.
"We know that Samuel's case was not an isolated one and the computer system that failed him put others at risk," Mr Starr said.
"Freedom of information requests have revealed that there were 63 missed paediatric cardiac appointments, with some taking nearly two years to discover.
"One of these was Samuel's."
Samuel's parents are now seeking independent advice to ascertain the effect of the delay on their son's condition.
"We believe that the system at Bath failed him and there were mistakes made at Bristol," Mr Starr continued.
"It was devastating to hear that an improperly implemented appointments system and a series of human errors resulted in the death of our son.
"We accept that mistakes happen but we believe that leaving a child unmonitored for as long as Samuel was, with so many opportunities to attend to him, goes beyond simple error."
Doctors diagnosed that Samuel had a restricted pulmonary artery - causing blood to easily flow away from his heart to his lungs - during a 20-week scan in pregnancy.
He was born at 39 weeks, on June 15 2009, weighing 8lbs. Despite his heart problem, Ms Holley told the inquest Samuel flourished and required little medical intervention.
His parents were warned Samuel would undergo a series of operation by the time he was aged five. He underwent open heart surgery for the first time on March 3 2010 at Bristol.
He was discharged six days later, and referred to the Paediatric Cardiac Clinic at the Royal United Hospital (RUH) in Bath for check-ups.
In October 2010, Samuel had his first check-up, in which an echocardiogram, also known as an "echo", was carried out.
His parents expected a second echo to be carried out early in the new year but during Samuel's next appointment, in April 2011 , one was not performed by p aediatric cardiologist Dr Andrew Tometzki .
Dr Tometzki ordered a further review - involving an echo - to be carried out in nine months time.
However, Samuel's parents were left chasing the appointment along with community children's nurse Clare Mees, who repeatedly phoned Dr Tometzki's medical secretary Annabelle Attridge.
Ms Attridge insisted she had taken details and forwarded them on to a dedicated appointments team - but "glitch" problems with the new bookings system meant the appointment was not logged.
Coroner Ms Voisin said: "This overall failure led to a five month delay in Samuel being seen by his cardiologist. This meant Samuel had not been seen for five months and had not had an echo for 20 months."
Samuel did not have the vital check up with Dr Tometzki until June 21. The consultant immediately ordered heart surgery due to the deterioration in his condition.
He underwent surgery at the Bristol Royal Hospital for Children on August 6 2012. As he was brought out of sedation on August 9, Samuel suffered a stroke and cardiac arrests.
The coroner said expert opinion was that as time went on "the more complicated the surgery is, as the more disadvantaged the heart is and the more difficult it is to get over the operation".
Samuel never recovered from the operation and on September 6, he died at 6.35pm in the hospital ward.
Ms Holley wept as she told the inquest: "As we read Samuel his favourite stories, he died in our arms."
The coroner said she would not make any recommendations to the hospitals, as changes had already been made.
But she ruled: "Due to the failure of the hospital outpatients booking system, there was a five month delay in Samuel being seen and receiving treatment.
"Samuel's heart was disadvantaged and he died following urgent surgery."
In a statement, Royal United Hospital Bath NHS Trust apologised to Samuel's parents.
"We offer our sincere condolences to the family of Samuel, and are very sorry for the delay in him being seen in the specialist outpatient clinic," a spokeswoman said.
"As the Coroner noted following her narrative finding, we have already taken action to correct any issues relating to the booking of outpatient appointments."
Sir Bruce Keogh, the medical director of NHS England, has ordered a review of children's cardiac services at Bristol Royal Hospital for Children (BRHC).