Hampshire Constabulary said "there are serious concerns" for the life of a five-year-old boy taken by his parents from hospital without the blessing of doctors as he needs constant medical care.
Police said Ashya King was taken from Southampton General Hospital at around 2pm yesterday by his family, who live in St David's Road, Southsea, Portsmouth.
Ashya is likely to be in a wheelchair or buggy, he cannot communicate verbally and is immobile, a police spokesman said.
Officers said his parents - Brett King, 51, and Naghemeh King, 45 - boarded a cross-Channel ferry from Portsmouth to Cherbourg at 4pm yesterday with Ashya's six siblings.
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.