The parents of Ashya King are being held in Spain after their extradition hearing was adjourned to allow the judge time to translate a number of documents.
Brett and Naghemeh King can be detained for up to 72 hours.
Ashya King's parents told the Daily Mail they fled the UK after being told NHS treatments would leave him with life-long disabilities.
The head of the police force which applied for a European Arrest warrant against the parents of Ashya King has now urged their release.
A British mother whose son's life was saved by proton therapy has told ITV that she defied NHS doctors' advice to pursue the treatment.