"Time is running out" for a boy with a brain tumour who was taken from hospital by his parents, British police say.

Hampshire Constabulary said "there are serious concerns" for the life of Ashya King (5) as he needs constant medical care.

At a news conference, officers revealed that Ashya had been operated on seven days ago, and had been fed through a tube in his hospital bed.

Assistant Chief Constable Chris Shead said: "The feeding system is battery operated and that battery will run out today.

"It is vital that we find Ashya today. His health will deteriorate rapidly."

He added: "Time is running out for this little boy. We need to find him and we need to find him urgently."

"The information we have received from his medical team at Southampton General Hospital is that he must continue to be fed via a tube by someone with the relevant medical training. If he doesn't receive urgent medical care, or the wrong treatment is given, his condition will become life-threatening."

Assistant chief constable Shead says he does not know if a crime has been committed.

Ashya was taken from his ward bed at around 2pm on Thursday. CCTV images captured him being wheeled from the hospital by his father Brett King.

Mr King (51) and his wife Naghemeh King (45) boarded a cross-Channel ferry from Portsmouth to Cherbourg at 4pm with Ashya and six siblings.

Mr Shead said the six-and-a-half-hour gap between Ashya being taken from hospital and police being called would be looked at "further down the line".

Appealing directly to the family, Mr Shead said: "Our message to you is 'please take Ashya to the nearest hospital immediately'. We understand this must be an awful time for you but the most important thing is to get the proper medical care for Ashya."

"Please work with us to provide Ashya that care."

CCTV footage shows Ashya in the hospital with his father

Mr Shead said he was unable to confirm whether a return ferry ticket had been booked, saying it was a line of inquiry.

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

The family, believed to be Jehovah's Witnesses, are travelling in a grey Hyundai I800 Style CRDI, registration KP60 HWK.

They arrived in France at roughly 8pm local time on Thursday and are still thought to be in the country.

Hampshire Police are working with officers in France to activate their emergency child rescue alert procedures to locate the family.

A Southampton hospital spokesman said police have been provided with a detailed medical report on Ashya so any hospital he is taken to can provide care.

The spokesman said Ashya was a long term patient who was permitted to leave the ward under the supervision of his parents as part of his ongoing rehabilitation.

"When the length of time he had been absent became a cause of concern to staff yesterday afternoon they contacted police after a search of the site and attempts to contact the family were unsuccessful," the spokesman said.

In a video posted on YouTube last month, Naveed King said his little brother Ashya had been diagnosed with a brain tumour and was undergoing emergency surgery.

Speaking into the camera in a message directly to Ashya, he said "I haven't slept. I've been awake all night worrying. We love you so much. We're all here for you. Everyone is praying for you. We just want to see your smile again."

"No kid at the age of five deserves to have a brain tumour. Let's just hope the doctors know what they're doing and they know exactly where to operate and what to take out, and they take everything out and you can be better."