Ashya King's parents, who are also British, were arrested yesterday in southeastern Spain after a European arrest warrant was issued by Interpol at the request of British police.
Their son is receiving medical treatment for a brain tumor, and after his parents' arrest he was admitted to a Spanish hospital.
The family has criticised Britain's health care system, saying he needs an advanced treatment option called proton beam therapy and that it wasn't being made available to him.
Judge Ismael Moreno was expected to interview the parents and rule on whether they should go free or be held in jail while the court decides whether to extradite them, the spokesman said. He spoke on condition of anonymity because he wasn't allowed to be cited by name in the media.
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British police say the parents, Brett and Naghemeh, are suspected of neglect. They are both Jehovah's Witnesses, but there has been no indication they raised any religious issue about the boy's treatment.
The family had been seen traveling from Britain to France aboard a car ferry and Spanish police had been alerted.