The parents of Ashya King put the young cancer patient "at risk" by taking him abroad without consent, according to a new report.
Brett and Naghmeh King sparked an international manhunt when they took Ashya from Southampton General Hospital in August 2014 and left for a proton beam clinic in Prague.
The pair were arrested in Spain and spent several nights in prison before being released, amid initial fears over the boy's welfare.
The report by Portsmouth Safeguarding Children Board analysed hospital records at the time, and found that while staff had worked hard to achieve a partnership with the parents, this had "deteriorated over time".
"This resulted in the parents removing their child from hospital, without discussion with medical staff, in order to take him abroad where they thought they would be able to access the services that they considered best met his needs," it said.
It then claimed: "This action put him at risk."
The report also criticised hospital staff for a delay in obtaining a second opinion for the parents, a factor it claimed had been "relevant" in their decision to flee.
It added that there were "also some concerns about the parents' actions in criticising and disregarding nursing advice which needed to be addressed more directly."
Eventually, the Kings were given permission to go to Prague, with the NHS agreeing to pay for Ashya's treatment.
Ashya has since been been declared cancer-free, and has returned to the UK with his family.
After his successful treatment, Brett King claimed the family's actions had saved his life, while his mother told the Sun: "If we had left Ashya with the NHS in Britain, he would not be with us today. He was too weak and would not have survived."