An Auckland mother at the centre of a British legal battle over treatment of her young son has learned his cancer has returned.

Sally Roberts, 37, originally from New Zealand, went into hiding last week to stop doctors treating her son, Neon Roberts, seven, with radiotherapy.

The National Health Service got a court order to have him treated but Roberts went into hiding with him.

They were found by police and taken before a special High Court sitting but Justice Bodey, who had been expected to rule, said there had been a change overnight in the boy's "ability to be treated with such therapies at this present time".

According to the Mail on Sunday, the family said Neon's brain tumour had returned.

The Mail said the family were given the devastating news on Friday, meaning any legal ruling had to be delayed.

It quoted the boy's grandfather Christian Roberts saying the diagnosis came after Neon underwent an MRI scan.

"I heard from my son on Friday, who took him for a scan, that the cancer is back," he said.

"I support my son. I want Neon to have whatever treatment is going to make him better."

Asked if this includes radiotherapy, the 68-year-old replied: "Yes."

In court, Roberts apologised for hiding her son, saying she feared that if she took him to hospital, they would go ahead with treatment.

"I wish the best for my son and the best quality of life, and feel radiotherapy could damage and greatly affect his future."

She said radiotherapy would reduce her son's IQ and could restrict his growth, as well as damage his thyroid and possibly leave him infertile.

Neon began suffering headaches and a swollen neck earlier this year and a brain scan in October showed a malignant tumour in his cerebellum, just above the neck.

After nine hours in surgery, a scan suggested the tumour had gone.

But the return of the tumour means any court decision will be delayed with Justice Bodey saying medical developments had occurred.

"This has changed the medical landscape. Nature is no respecter of the courts."

- © Fairfax NZ News