An American specialist who has offered to treat Charlie Gard is expected to examine the terminally-ill baby for the first time over the next two days.
Michio Hirano, a professor of neurology at the Columbia University Medical Centre in New York, is scheduled to visit Charlie at Great Ormond Street Hospital in London and discuss the case with doctors treating Charlie plus other medical experts.
The visit has been organised as part of the latest stage of a court fight over whether 11-month-old Charlie should be given experimental treatment in America.
Chris Gard and Connie Yates want a judge to rule that their son, who suffers from a rare genetic condition and has brain damage, should be allowed to undergo a therapy trial overseen by Dr Hirano in New York.
Specialists at Great Ormond Street, where Charlie is being cared for, say the therapy is experimental and will not help.
They say life support treatment should stop.
Charlie's parents, who are in their 30s and come from Bedfont, west London, have already lost battles in the High Court, Court of Appeal and Supreme Court in London.
They have also failed to persuade European Court of Human Rights judges to intervene.
But the couple say there is new evidence and want Mr Justice Francis to carry out a fresh analysis of their case.
In April, the judge ruled in favour of Great Ormond Street and said Charlie should be allowed to die with dignity.
Mr Justice Francis has considered the couple's latest claims at preliminary hearings in the Family Division of the High Court in London over the last few days.
He is due to stage further hearings later this month following Dr Hirano's visit.