Police say Ashya King’s life will be at risk if they do not find him today.
Hampshire Constabulary has launched a major search for the five-year-old Southsea boy, who yesterday was taken by his parents from the hospital where is was undergoing treatment for a brain tunour.
The family later boarded a France-bound ferry in Portsmouth.
A police spokesman said: “We are working with our counterparts in France to activate their emergency child rescue alert procedures to locate the family as soon as possible.
“If we do not locate Ashya today there are serious concerns for his life. He is receiving constant medical care within the UK due to recent surgery and ongoing medical issues.
“Without this specialist 24-hour care Ashya is at risk of additional health complications which place him at substantial risk.”
Ashya’s brother Naveed published an emotive video on YouTube after doctors made the brain tumour dignosis.