The five-year-old cancer patient, Ashya King, whose parents sparked an international manhunt when they took him from hospital in Britain, left Spain for treatment in Prague.
A medical plane carrying Ashya took off from Malaga in southern Spain for Prague after health and legal authorities finally sealed the formalities for his transfer, the Spanish hospital that treated him said in a statement.
The doe-eyed boy has been in the middle of a legal saga that began when his desperate parents took him out of a British hospital against doctors' advice on August 28.
That triggered a cross-border manhunt that saw the parents jailed for four days in Spain before being freed and emotionally reunited with Ashya at his bedside in the Maternity and Children's Hospital, Malaga.
The case dominated British news, with public opinion shifting from outrage to sympathy for parents Brett King, 51, and his wife Neghemeh King, 45.
The couple said they took their son abroad to avoid radiotherapy treatment that they believed would turn him into a "vegetable". They argue that such treatment is too strong for a child.
British prosecutors at first suspected them of cruelty but later dropped the case, acknowledging that Ashya had been properly looked after.
Before moving him from hospital in Spain they had to wait for a British court to give up its legal custody of Ashya, which had been imposed after they were detained.
Ashya, who has undergone surgery for a brain tumour, is now headed to a Czech centre specialising in proton beam therapy, a treatment that was unavailable to him in Britain.
- Proton beams -
Proton beam therapy is said to be more precise than traditional radiotherapy, allowing doctors to deliver higher doses of energy to a tumour while better sparing surrounding healthy tissue.
Peter Wilson, lead paediatrician at Southampton hospital in southern England where Ashya was treated for his tumour, did not believe that the alternative therapy would help to rid the boy of the type of tumour he has, however.
The Proton Therapy Centre (PTC) in Prague said its experts would fast-track their procedures for the young patient, who was expected to arrive in Prague early Monday afternoon.
"He will be transferred by ambulance to our oncology and haematology clinic for initial tests," Miloslav Ludvik, director of the Prague hospital, told journalists on Monday.
Jan Stary, head of the children's haematology and oncology clinic, said Ashya could begin to receive the therapy next week at the earliest.
He estimated that Ashya had a 70 percent chance of survival if all the treatment went well.
According to the PTC, the procedure costs about 1.8 million kroner (65,000 euros, $84,000) in the Czech Republic, compared with 108,000 euros in the United States.
The Kings, who are Jehovah's Witnesses, have said they will sell a home they own in Malaga to fund Ashya's treatment. Donations are also reported to have flooded in from wellwishers.
The couple's legal troubles prompted an outpouring of public support in Britain, where tens of thousands of people signed a petition calling for the boy to be reunited with his parents.
The case even gained the attention of Prime Minister David Cameron, who upon learning that the case against the Kings had been dropped, tweeted: "It's important this little boy gets treatment and the love of his family."