A nationwide hunt is under way after a mother and her "seriously ill" seven-year-old son, who is in need of cancer treatment, are thought to have disappeared.

Sally Roberts, also known as Sally Leese, and her son Neon, are thought to have left their home town of Tiverton, Devon, on December 3.

Devon and Cornwall Police launched an urgent appeal after growing concern for the health of the youngster, who has recently undergone surgery for a brain tumour.

Doctors treating him said he is in need of urgent medical attention, a spokesman for the force said. It is believed his mother is opposed to further treatment and has disappeared with him.

Their whereabouts is currently unknown, but police are asking any members of the public who may have seen them or know where they are to come forward.

Assistant chief constable Chris Boarland said: "Neon is seriously ill and we have huge concern for his welfare. He is currently subject to a Family Court order and I would ask his mother to think about her son's welfare and contact police and medical services as soon as possible.

"Neon needs urgent medical attention and his wellbeing, along with his mother, is our prime concern. If any member of the public has seen Neon and Sally or knows their whereabouts they should contact police as a matter of urgency on 0800 0560154."

According to the Daily Mail, Ms Roberts is originally from New Zealand and her former partner and Neon's father Ben, an IT consultant from Knightsbridge, London, has not heard from either of them since the weekend.

The newspaper reported that at a High Court hearing on Wednesday, lawyers for the NHS, Devon County Council and "those tasked with looking after the boy's best interests agreed his disappearance should be made public".

The mother and son are believed to have been seen in the Sussex/Kent area. Devon and Cornwall Police has set up a dedicated Twitter hashtag for anyone with information to help the investigation at #FindNeon to @DC_Police.