Interpol issued a yellow notice alert for people to help find Ashya King, five, and his family.
His parents Brett King, 51, and his wife Naghemeh, 45, of Southsea, Hampshire, who are Jehovah's Witnes-ses, took him from Southampton General Hospital at around 2pm on Thursday without the consent of doctors and caught a ferry to Cherbourg two hours later.
The battery on the wheelchair-bound Ashya's feeding system, which allows him to consume liquids through a tube, was due to run out last night.
Assistant Chief Constable Chris Shead said: "Ashya is in a wheelchair and is fed through a tube. The feeding system is battery-operated and that battery will run out. Time is running out for this little boy. We need to find him and we need to find him urgently."
Ashya has undergone extensive surgery and his last operation was seven days ago. However, they have admitted there was a six-and-a-half-hour gap between Ashya being taken from hospital and police being called. This would be considered "further down the line".
Mr Shead said: "The information we have received from his medical team 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."
The family, who live in St Davids Road, Southsea, Portsmouth, are travelling in a grey-coloured Hyundai I800 Style CRDI, registration KP60 HWK.