"Time is running out" for a five-year-old boy with a brain tumour who was taken without consent from hospital by his parents as the battery on his feeding system runs out today, police have said.
Ashya King's family took him from Southampton General Hospital at around 2pm yesterday and travelled on a ferry to France some two hours later.
Hampshire Constabulary said they were told by the hospital that the youngster was missing at 8.35pm last night - more than six hours after he had been taken by his parents, Brett (51) and Naghemeh King (45).
Assistant Chief Constable Chris Shead said: "It is vital that we find Ashya today. His health will deteriorate rapidly."
He told a press conference: "Ashya is in a wheelchair and is fed through a tube. The feeding system is battery operated and that battery will run out today.
"Time is running out for this little boy. We need to find him and we need to find him urgently."
Hampshire Constabulary have launched a "major investigation", saying there are serious concerns for the boy's life as he needs constant medical care.
Mr Shead said Ashya had undergone "extensive surgery" and his last operation was seven days ago.
"The information we have received from his medical team at Southampton General Hospital is that he must continue to be fed via a tube by someone with the relevant medical training," he said.
"If he doesn't receive urgent medical care, or the wrong treatment is given, his condition will become life-threatening."
Ashya's parents boarded a ferry from Portsmouth to Cherbourg at 4pm yesterday with his six siblings and arrived in France at 8pm local time, police have said.
The family, who live in Southsea, Portsmouth, are travelling in a grey Hyundai I800 Style CRDI, registration KP60 HWK.
Ashya is likely to be in a wheelchair or buggy, he cannot communicate verbally and is immobile, a police spokesman said.
Mr Shead said police were keeping "an open mind" about the parents' motive for taking their son from hospital and refused to comment on whether they were Jehovah's Witnesses.