Five-year-old Ashya King was recovering from surgery at Southampton General Hospital when his parents, Brett King, 51, and Naghemeh King, 45, removed him from the facility Thursday.
Hospital officials said they're concerned for the boy's health and safety because a feeding system he was hooked up to is likely to have run out of battery power.
"It is vital that we find Ashya today, his health will deteriorate," Assistant Chief Constable of Hampshire Chris Shead told BBC. "If he doesn't receive urgent medical care, or the wrong treatment is given, his condition will become life-threatening."
Police believe the Kings were taking Ashya to Marbella, Spain.
The family, including Ashya's six siblings, boarded a ferry from Portsmouth to Cherbourg, France. Police said Ashya is unable to walk and is likely in a wheelchair or stroller.
"They may have stayed in Cherbourg, but they may be 500km (310 miles) from here, it's like looking for a needle in a haystack," said vice-prosecutor Cyril Fournier.