Brain cancer patient Ashya King's parents "put him at risk" by removing him from hospital without consent and taking him abroad for treatment, a report has found.
Brett and Naghmeh King sparked an international police hunt when they removed their son from Southampton General Hospital in August 2014 to take him to Prague for proton beam therapy.
A report by Portsmouth Safeguarding Children Board into how agencies dealt with the case said analysis of records by hospital staff showed they clearly worked hard to achieve a partnership with the parents, but "nevertheless there is clear evidence that this relationship 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 added.
"This action put him at risk."
The report said the question must be asked about whether there was any way in which the breakdown in trust could have been avoided.
One factor that is relevant was a delay in obtaining a second opinion for the parents, it said.
"Whilst the doctors' view that this was not needed immediately was accurate in terms of the child's clinical needs; this failed to take account of the indirect message that was given to the parents, which was that their wishes and rights were overruled by the professionals.
"There were also some concerns about the parents' actions in criticising and disregarding nursing advice which needed to be addressed more directly."
It suggested a formal meeting could have been held to discuss directly the worries that health professionals had about their willingness to accept guidance.
The report also highlights "the limitation of the current legal processes available to professionals working with children and families in these circumstances".
It points out that once the parents were arrested, the matter was managed by the Spanish judiciary and no UK agency had direct influence on the decision-making.
"Thus the only mechanism that was speedy enough to respond to the concerns was one where any discretion about the nature of the intervention was passed to a third party, the Spanish Police and Judiciary."
The report adds: "Once the parents had removed the child from the hospital there were limited options available to the agencies as there were real concerns that he was at immediate risk of significant harm.
"These concerns were partly a result of the parents concealing the actions they had taken to ensure his safety and were compounded by them failing to respond to attempts to contact them.
"The legal options available to agencies were draconian and did not allow for any flexibility in application."
Last week the woman in charge of the medical centre in Prague that treated Ashya said she is "over the moon" with his progress.
Iva Tatounova said the six-year-old's recovery was more proof that proton beam therapy should be "the first choice" of treatment for children with his condition.
Ashya's parents took their son back to the centre in the Czech capital, one year after they removed him from the hospital in Southampton.
Ms Tatounova, director of the Proton Therapy Centre, said: "He's doing fantastically. I'm over the moon actually. He walked to the centre on his own feet. He didn't need a wheelchair.
"He's speaking and playing with other kids and his siblings, and next week (he's) going, for the first time, to a school."
She said proton beam therapy - which is not widely available in the UK - "should be the first choice of treatment" for young children with brain cancer "because it's less aggressive".
Police launched an international search for the King family after Ashya was removed from the hospital.
Mr and Mrs King were arrested a few days later in Spain, where they were forced to spend several nights in prison away from their son before being released.
A High Court judge later approved the move to take Ashya to Prague for proton therapy.
The treatment limits the collateral damage of radiation to other vital organs, such as the heart and liver in Ashya's case, and can lead to less severe long-term side-effects including heart and breathing problems.
The therapy was not offered to Ashya on the NHS, although the health service later agreed to fund his treatment. Several new proton beam therapy centres will open in the UK from next year.
Ashya's grandmother Patricia King has previously described the authorities' handling of the case as a "huge injustice".