Great Ormond Street Hospital has said clinical staff are "on hand to facilitate the visit" and Dr Hirano has been given an honorary contract, giving him full access to the baby's medical records.
Columbia University medical school specialist Michio Hurano on Monday examined Charlie Gard with British colleagues ahead of a High Court hearing on new experimental treatment and a possible move to the U.S. for the terminally ill 11-month-old British boy, whose Great Ormond Street doctors are poised to pull the plug after earlier British and European court rulings.
Hirano was invited to make the visit after offering an experimental gene therapy he believes has a 10% chance of improving muscle strength and bringing a "small but significant" improvement to Charlie's brain function.
He further noted that the small number of people with Charlie's rare genetic condition, mitochondrial DNA depletion syndrome, would make robust clinical trials hard.
But Britain's courts have refused permission on the grounds it would prolong his suffering without a realistic prospect of it helping the 11-month-old child.
Great Ormond Street Hospital requested a new hearing after the New York-based neurologist claimed his treatment could now have a better chance of working.
United Kingdom doctors have argued that he should be allowed to die but his parent's Chris Gard and Connie Yates see the specialist as the last hope for Charlie.
The judge ruled that Charlie's mother Connie, from Bedfont, west London, can be present for the examination.
They say life support treatment should stop.
But the couple say there is new evidence and want Mr Justice Francis to carry out a fresh analysis of their case.
The European Court of Human Rights also decided not to intervene in the case.
In April, the judge ruled in favour of Great Ormond Street and said Charlie should be allowed to die with dignity.
The hospital added it had treated more than 1,000 patients with mitochondrial disease and offered pioneering treatment, including nucleoside treatment, where appropriate.
Preliminary hearings were held at the Family Division of the High Court in London over the last week.
He is due to travel to London on Tuesday to examine Charlie.