Deputy Prime Minister Nick Clegg joins mounting calls for Ashya King to be reunited with his jailed parents as Portsmouth City Council calls for a halt to extradition proceedings against them.
Brett King and his wife Naghemeh, from Portsmouth, remain in police custody in Madrid while a Spanish court considers whether to grant a British extradition request.
A hearing will be held at the family court this afternoon.
Ashya, who has a brain tumour, is under police guard at the Materno-Infantil hospital in Malaga, about 330 miles south of the capital.
He has not seen his parents since they were arrested on Saturday after taking Ashya out of Southampton General Hospital to seek specialist cancer treatment abroad.
Ashya's parents arrive at court on Monday
The five-year-old's six siblings are also being prevented from visiting him at the hospital.
Nick Clegg told Sky News it was a "heartbreaking" situation.
"I've got a five-year-old son and the idea of leaving him in a hospital with no contact with parents and siblings fills me, as I imagine it fills all parents, with horror," he said.
"I would like to see the family reunited and then hopefully people can calmly make a decision about what should happen next.
"Throwing the full force of the law at two parents who in a state of despair and anguish are acting they say because that's what they think is best for their child, doesn't seem to me to be the appropriate thing to do."
Meanwhile Portsmouth City Council Leader, Donna Jones, issued a statement calling on the Crown Prosecution Service to review the case.
Naveed King, Ashya's brother
"Like others who have been watching this upsetting case unfolding in the media, I have been moved by the plight of the King family and am most concerned about Ashya. I believe what he needs now is to be with his family," she said.
"That's why I am urging the CPS to urgently review the case involving Ashya King's parents and remove any extradition proceedings, so the family can be reunited with their five-year-old son."
A petition calling for the family to be reunited has been handed in to Downing Street.
It has emerged that Ashya's parents are planning legal action against Southampton General Hospital.
Juan Isidro Fernandez Diaz, the couple's lawyer in Spain, said they are "so sad", adding: "They are going to prepare legal demands against the hospital in Southampton. Legal action will be against the hospital."
A spokesman for the hospital said at the weekend that "our priority has always been Ashya's welfare".
Ashya and Brett King in a video the family posted online
He added: "Throughout Ashya's admission we have had conversations about the treatment options available to him, and we had offered the family access to a second opinion as well as assistance with organising treatment abroad."
Former children's minister Tim Loughton wrote on Twitter that the Crown Prosecution Service should drop the case against the family, called on the police to apologise and for the NHS to fund treatment for Ashya.
Meanwhile, British police officers are understood to have arrived in Spain to question Ashya's parents.
Hampshire Police's Assistant Chief Constable Chris Shead said he was aware the police's approach had led to a "significant amount of debate" but he would rather be criticised for "being proactive" than "potentially having to explain why a child has lost his life".
Simon Hayes, Police and Crime Commissioner for Hampshire and the Isle of Wight, said: "Hampshire Constabulary's role, as in many other cases, was to safeguard the interests of a very vulnerable sick young child and find Ashya."
The Crown Prosecution Service said the case was under "immediate review" and a decision on whether to prosecute would be made.
Ashya's grandmother Patricia King has accused the authorities of treating the couple "like murderers".
Patricia King earlier said her son was selling his holiday home in Spain to pay for proton beam therapy, which costs an average of £100,000 per person.