The parents of 12-year-old Archie Battersbee say it is "unfair" that they are having to fight for their son's life just four weeks after a catastrophic injury left him unconscious.

A High Court judge ruled on Friday that doctors at the Royal London Hospital can go ahead with a brain stem test to assess his condition. 

They believe Archie is brain dead and if he fails the test, they will seek to have his life support withdrawn. 

The judge determined that a brain stem test would be in Archie's best interests.

His parents Hollie Dance and Paul Battersbee say that it is "too soon" to decide whether life support should be removed. 

"I am trying to be there for my little boy while also fighting a legal battle, it is so difficult and unfair," said Ms Dance.

"Archie had a severe brain injury only four weeks ago, there's not been enough time to see what he can do."

Ms Dance said that in the month since her son's hospitalisation, he had squeezed her fingers "with a tight grip" and that "tears had appeared in his eyes" as a ventilator tube was being replaced.

"I think that's his way of letting me know he's still here and just needs more time," she said.

At a private hearing in the Family Division of the High Court in London this week, their lawyers from the Christian Legal Centre argued that brain stem tests are are not sufficient to make an assessment of Archie's condition. 

"Until it's God's way I won't accept he should go. I know of miracles when people have come back from being brain dead," Ms Dance continued.

"He may not be the same as he was but if there's a possibility he could live a happy life after this, I want to give it to him."

Andrea Williams, chief executive of the Christian Legal Centre, said, "What happened to Archie because of his accident is a tragedy. However, it will be an even greater tragedy should he be removed from ventilation when there is still hope.

"We are fighting to give Archie every chance to recover.

"This case raises the significant moral, legal and medical question as to when a person is dead. Archie's parents do not accept that he is dead and are fighting for his life."