Interpol has issued a global alert for a desperately ill boy with a brain tumour who officials believe was kidnapped from hospital by his parents, dramatically ratcheting up the hunt for the five-year-old.

"At the request of UK authorities Interpol has issued a Yellow Notice, or missing persons alert, for a five-year-old British boy removed from a hospital by his parents," the international police organisation said in a statement.

Earlier, UK police said they are working with their French counterparts to find a five-year-old boy with a brain tumour who was taken away from hospital by his parents.

Ashya King's parents took him from a hospital in Southampton, southern England, yesterday without doctors' consent and along with his six siblings boarded a ferry to the French port of Cherbourg the same day, a police spokesman said.

The spokesman said the boy's life was at "substantial risk" as he needed constant medical care following recent surgery, adding that he was likely in a wheelchair or buggy as he cannot move.

"We are working with our counterparts in France to activate their emergency child rescue alert procedures," he said.

Police said Ashya was taken from Southampton General Hospital at around 2pm yesterday by his family.

Ashya is likely to be in a wheelchair or buggy, he cannot communicate verbally and is immobile, a police spokesman said.

The family are travelling in a grey-coloured Hyundai I800 Style CRDI, registration KP60 HWK, and they are still believed to be in France.

Police said they arrived in France at 8pm local time yesterday.

It was unclear what motivated the parents to take their son away.