"Time is running out" for a five-year-old boy with a brain tumour who was taken from hospital by his parents, police say.
Hampshire Constabulary said "there are serious concerns" for the life of Ashya King as he needs constant medical care.
Interpol has issued a global Yellow Notice missing persons alert for the missing boy who police say was taken to France on Thursday.
CCTV captures Ashya being taken from the hospital in his wheelchair
The notice was issued at the request of UK authorities and has been circulated to all of Interpol's 190 member countries.
British police say Ashya was operated on seven days ago, and had been fed through a tube in his hospital bed.
Assistant Chief Constable Chris Shead told a news conference: "The feeding system is battery operated and that battery will run out today.
Interpol have joined the urgent hunt for missing Ashya
"It is vital that we find Ashya today. His health will deteriorate rapidly."
He added: "Time is running out for this little boy. We need to find him and we need to find him urgently.
"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.
Ashya needs 24-hour medical attention, say police. Pic Naveed King
"If he doesn't receive urgent medical care, or the wrong treatment is given, his condition will become life-threatening."
Ashya was taken from his ward bed at around 2pm on Thursday. CCTV images captured him being wheeled from the hospital by his father Brett King.
Mr King, 51, and his wife Naghemeh King, 45, boarded a cross-Channel ferry from Portsmouth to Cherbourg at 4pm with Ashya and six siblings.
The family boarded a ferry from Portsmouth to Cherbourg
They arrived in France at roughly 8pm
Police believe the family are still in France
Mr Shead said the six-and-a-half-hour gap between Ashya being taken from hospital and police being called would be looked at "further down the line".
Appealing directly to the family, Mr Shead said: "Our message to you is 'please take Ashya to the nearest hospital immediately'.
"We understand this must be an awful time for you but the most important thing is to get the proper medical care for Ashya.
"Please work with us to provide Ashya that care."
Ashya with his father in hospital. Pic: Naveed King
Mr Shead said he was unable to confirm whether a return ferry ticket had been booked, saying it was a line of inquiry.
The sick child is still likely to be in a wheelchair or buggy, he cannot communicate verbally and is immobile, a police spokesman said.
The family, believed to be Jehovah's Witnesses, are travelling in a grey Hyundai I800 Style CRDI, registration KP60 HWK.
They arrived in France at roughly 8pm local time on Thursday and are still thought to be in the country.
Hampshire Police are working with officers in France to activate their emergency child rescue alert procedures to locate the family.
Hampshire Police have launched a social media campaign to find Ashya
Meanwhile, Guy Canonici, the president of the Jehovah's Witnesses in France, has told Sky News he has put over 1,000 Kingdom Halls (places of worship) on alert for the missing boy.
He said so far no one has come forward with any information.
TV channels and newspaper websites in France have urged motorists to look out for the right-hand drive English car carrying Ashya
A Southampton hospital spokesman said police have been provided with a detailed medical report on Ashya so any hospital he is taken to can provide care.
The spokesman said Ashya was a long term patient who was permitted to leave the ward under the supervision of his parents as part of his ongoing rehabilitation.
Ashya with his mother Naghemeh. Pic Naveed King
"When the length of time he had been absent became a cause of concern to staff yesterday afternoon they contacted police after a search of the site and attempts to contact the family were unsuccessful," the spokesman said.
In a video posted on YouTube last month, Naveed King said his little brother Ashya had been diagnosed with a brain tumour and was undergoing emergency surgery.
Speaking into the camera in a message directly to Ashya, he said: "I haven't slept. I've been awake all night worrying.
"We love you so much. We're all here for you. Everyone is praying for you. We just want to see your smile again.
"No kid at the age of five deserves to have a brain tumour.
"Let's just hope the doctors know what they're doing and they know exactly where to operate and what to take out, and they take everything out and you can be better."
A car similar to the one being used by the family
Naveed, whose Instagram profile says he is 20, describes himself as a Jehovah's Witness on the social networking site.
Jehovah's Witnesses refuse blood transfusions on religious grounds but are open to other medical procedures.
Ashya's paternal grandmother, Patricia King, said his parents were "wonderful" and had been left beside themselves at their child's plight.
Brett King is understood to have wheeled the boy out of the hospital
Speaking from her home in Southsea, she said of her son: "He's the most caring and wonderful father you could ever have. The kids love him."
She said she last spoke to her son "quite a while ago", adding: "He wouldn't have told me anything because he wouldn't want me to know anything in case I got involved in it all."
She said she did not know whether Ashya's illness was terminal, saying: "I knew he was seriously ill, we all knew that."
Anyone with information about Ashya's whereabouts should contact Hampshire Constabulary on 101, quoting Operation Aquilion.