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Mr King said he had spoken to specialists following Ashya's surgery and had requested proton beam treatment, which was not available on the NHS.
''Proton beam is so much better for children with brain cancer,'' he said. ''It zones in on the area, whereby normal radiation passes right through his head and comes out the other side and destroys everything in his head.
''We pleaded with them for proton beam treatment. They looked at me straight in the face and said with his cancer - which is called medulloblastoma - it would have no benefit whatsoever.
''I went straight back to my room and looked it up and the American sites and French sites and Switzerland sites where they have proton beam said the opposite, it would be very beneficial for him.
''Then I spoke to them again, I wrote a letter which he never responded to, saying OK - I will sell my property in order to pay for the proton beam.''
Mr King said his son's treatment seemed like ''trial and error'' and he was told if he questioned the treatment the hospital would seek an emergency protection order.
He said: ''After that I realised I can't speak to the oncologist at all, because if I actually ask anything or give any doubt I wasn't in full accord with them, they were going to get a protection order which meant in his deepest, darkest hour I wouldn't be there to look after him, and neither would my wife - they would prevent us from entering the ward.
''That's such a cruel system I decided I to start looking at the proton beam myself.''
He added: ''We decided to try and sort it out ourselves but now we're refugees almost.
''We can't do anything. The police are after us. The things we want to do to raise the money to pay for the proton beam, they've prevented it now.
''So my son is being treated and he's doing fine. We're very happy with his progress. We're not neglecting him. He has everything he had in hospital.''
Mr King said Ashya was ''responding so much better'' than he did in hospital.
''We couldn't take it any more - not knowing and not being able to question anything in fear that they say, 'Sorry Mr and Mrs King, emergency protection order, you're no longer allowed in the ward','' he said.
''Under that stress, our son has grade four brain tumour, we couldn't discuss or question them at all in fear that our son would be in that ward all day long by himself without his parents being able to come in.
''We couldn't be under that system any more.
''I was going to get the money to pay for the proton beam therapy but they have prevented that now because the Spanish police are involved and I can't do want I wanted to do.''
Mr King urged police to call off ''this ridiculous chase''.
''We're not neglecting our son, he's in perfectly good health,'' he said.
''My son is smiling, he's happy, we're doing things as a family. We just want to be left in peace. He's very sick. I just want to get on with his treatment. I'm not coming back to England if I cannot give him the treatment I want, which is proper treatment.
''I just want positive results for my son.''
Spanish police were acting on a European arrest warrant requested by Hampshire Constabulary when they arrested the Kings.
When they stopped the family's Hyundai people carrier officers found Ashya and his parents inside.
Mr Shead said: "There are no winners in this situation. I've said all along that this must be a terribly distressing time for Ashya's family and I stand by that now."
He added that it was too soon to say when Ashya would come back to the UK but Southampton General Hospital have been contacted so they can liaise with the medical taking care of him in Spain.
"Ashya's brothers and sisters were not in the vehicle," Mr Shead said. "We have located them. They're all okay, they're fine. They are actually in a hotel about 10 miles away."
He also said that a team of Hampshire police officers would be going to Spain tomorrow to continue the investigation.
Meanwhile, in a video blog posted on YouTube by Ashya's brother Naveed, their father, a Jehovah's Witness, said Ashya was doing well and explained that the family had decided to take him out of hospital to seek a cancer treatment not available on the NHS.
Sitting on a bed with Ashya in his arms, Mr King said: ''We were most disturbed today to find that his face is all over the internet and newspapers and we have been labelled as kidnappers putting his life at risk, neglect.''
Mr King said there had been ''a lot of talk'' about the machine used to feed Ashya and whether they could make it work.
Police had warned that the family might not be able to work the machine and that it would run out of battery power.
Ashya King's family took him from Southampton General Hospital on Thursday afternoon and travelled on a ferry to France some two hours later.
Assistant Chief Constable Chris Shead of Hampshire Constabulary said the boy's parents Brett King, 51, and Naghemeh King, 45, had been arrested at 10pm local time after Spanish police stopped the family's vehicle.
"We don't have many details on Ashya's condition at this point in time but what we do know is that he was showing no visible signs of distress," Mr Shead said.
"Ashya has now been taken to a hospital in Malaga. The parents have been arrested. They have been taken to a police station."
Residents from a town in Hampshire joined together in an attempt to break the record for the longest chain of ice bucket challenges.
More than 100 people took to the high street in Leigh Park to take their turn to be drenched for a good cause.
This ice bucket challenge was organised by Andy's Army - in order to raise money for Andy Prowting, who has been diagnosed with cancer that the NHS can not treat.
Mr Prowting has been told he needs to make the life-saving trip to America - but at more than £50,000 it is not a treatment he can afford so he was turned to furious fundraising efforts.
Hampshire Constabulary is releasing more information about missing five-year-old Ashya King, including an image of Ashya's distinctive feeding pump.
Assistant Chief Constable Chris Shead said: "Health experts tell us that with the proper medical care Ashya has a good chance of survival, but he must get that professional care."
– Hampshire Police spokesperson
"The feeding system that Ashya needs and the other associated medical care is complex, and I would appeal to Ashya’s family not to think that they are able to administer this care themselves. "The doctors assure me that proper medical training is required for the level of care that Ashya needs. "I would also appeal to the medical profession, particularly in Spain, to contact the Spanish authorities if they feel that Ashya’s family have presented or made enquiries at medical facilities. "Whilst our primary focus is still on finding Ashya, we can confirm that we have obtained a European Arrest Warrant which will assist us in speaking to Ashya's parents when we find them. Our message to Brett and Naghmeh is that this should not deter you from coming forward, please just come and speak to us."
Hundreds of people took to the streets of Reading today in a colourful display of solidarity for the town's annual Pride parade. The event, which is in its eleventh year, is a celebration of the community's diversity and a march for equality.
Martin Dowse sends this from the festival.
Record numbers of rare Sand Martins have appeared at a nature reserve in Sussex. Conservationists at the Arundel Wetland Centre built an artificial colony for the birds to nest in. Now, large flocks of the Sand Martins have arrived to check out the site.
Malcolm Shaw has the story.
The hunt for gold continues today on a Kent beach as the first diggers to uncover the buried treasure have been announced.
A German artist sparked the 'gold rush' on Folkestone Harbour, where hundreds of people with buckets, spades and metal detectors have descended on the beach.
Artist Michael Sailstorfer has hidden 30 bars of pure gold under the sand as part of a public art festival.
Her work is usually sold in galleries fetching thousands, if not millions, of pounds. But today Margate's Tracey Emim was among many high-profile artists selling their work at a car boot sale as part of the Triennial in Folkestone.
Andy Dickenson reports and speaks to artists Jim Moir, Gavin Turk, Julia Riddiough and organiser of the Art Car Boot Fair, Karen Ashton.