(CNN) -

The British couple arrested after pulling their cancer-stricken son out of a hospital will not be charged, the Crown Prosecution Service told CNN on Tuesday.

Brett and Naghmeh King were arrested over the weekend in Spain while authorities decided whether they would be charged and extradited back to Britain.

On Monday, they said they would not return to the UK voluntarily, according to a Madrid court at which they appeared.

The Kings say they had serious concerns about the medical treatment Ashya, 5, was receiving at University Hospital Southampton. So they defied doctors' orders and took him out.

Hotel staff members in Malaga, Spain, recognized Ashya and his family from media coverage and contacted police. Ashya was taken to Materno Infantil Hospital in Spain, and that facility was communicating with University Hospital Southampton. The parents were taken into police custody.

Brett King defended his actions in a YouTube video made shortly before his arrest. He accuses two doctors at the British hospital of not allowing him to seek proton beam treatment outside of Britain, even though he said he was ready to pay for the treatment himself.

"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." King said he then looked on the Internet and found sites in the United States, France and Switzerland on proton beam treatment that "said the opposite -- that it would be beneficial for him."

The Kings traveled to Malaga with their six children to sell a home they own there, in order to "look for better treatment" for Ashya, said their Spanish lawyer, Juan Isidro Fernandez.

The hospital in Southampton issued a statement saying doctors had discussed proton beam radiotherapy with Ashya's parents.

"We very much regret that the communication and relationship with the King family had broken down in this way and that for whatever reason they have lost confidence in us," said Dr. Michael Marsh, medical director at University Hospital Southampton. "Our first concern is for Ashya's welfare"

Marsh said some tumors respond well to proton beam treatment, but "there are some cases where there isn't the evidence that this is a beneficial treatment." The hospital statement did not give specifics about Ashya's case.

The 5-year-old's ordeal has captured the attention of British Prime Minister David Cameron.

"I welcome the prosecution against Ashya King's parents being dropped," he said Tuesday on Twitter. "It's important this little boy gets treatment and the love of his family."

In the United States, only a handful of hospitals offer proton-beam therapy after surgery, including Massachusetts General Hospital, where the late U.S. Sen. Ted Kennedy was treated for brain cancer in 2008. He died just a year after his surgery for malignant glioma.

That surgery was followed by six weeks of radiation. Kennedy wrote about his experience in a Newsweek magazine article at the time that he underwent proton-beam therapy.

The theory behind proton therapy is that its high-energy particles home in on the tumor and so do not harm the surrounding healthy tissue as much as the X-ray photons in conventional therapy, said Dr. Donald O'Rourke, associate professor of neurosurgery at the University of Pennsylvania School of Medicine.