The police said they had "positive information" to suggest that Ashya King and his family could be in Spain where they have "strong links" to the Marbella area.
King's parents, who are Jehovah's Witnesses, drove him away from Southampton hospital in southern England on Thursday and were seen boarding a ferry to Cherbourg in France.
British and French police have been searching for the boy and a French prosecutor said the youngster had effectively been "kidnapped".
Interpol has also launched a missing persons alert following a request from British police, who warned that the batteries on King's feeding system would run out on Friday.
"The need to find Ashya is now desperate. It is really important that we find him and ensure he receives medical attention," said Chris Shead, deputy police chief in Hampshire, the region where the boy was taken from.
"We still don't know whether the King family have any spare batteries for the machine which administers food, the knowledge or any way of recharging the battery," Shead said.
The youngster has undergone extensive surgery and was last operated on seven days ago.
Jehovah's Witnesses are known for refusing blood transfusions on religious grounds, but are otherwise open to other medical procedures.
The group said there was no indication that the boy's parents had been motivated by religious convictions and appealed to them to seek the best medical treatment for him.
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