Hampshire Police will send a team of officers to the Spanish city of Malaga to investigate why Ashya King was removed from hospital by his parents.
Assistant Chief Constable Chris Shead told reporters: "Tomorrow morning we will be sending a team of officers to Malaga, they will liaise with our Spanish counterparts and we will continue the investigation from that point onwards."
"We now need to speak to those parents at the appropriate time to detemine exactl ywhat was the motivation behind them taking Ashya," he added.
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.