The neurologist said the treatment, known as nucleoside therapy, had been shown in mice to extend life span. He estimated the chance of clinical improvement of muscle function — defined as not having to constantly use a breathing machine — at 10 percent, but he acknowledged that the estimate derived from his work with patients with a mutation known as TK2, not the rarer mutation, RRM2B, that Charlie has.

Defying the consensus of British doctors, the neurologist said he could not be certain how much structural brain damage the baby has. He admitted, however, that he had not seen the boy, or reviewed his full medical records or even the extensive legal record in the case.

Pressed by Victoria Butler-Cole, a lawyer for the infant’s appointed guardian, the neurologist also acknowledged that the disease had no cure. He insisted, however, that the therapy had a small chance of improving the baby’s cognitive function — even though he told the court in April that the boy had suffered significant brain damage, and even though he acknowledged that the child’s condition had only worsened since then. (The neurologist said he may have “overstated” the damage.)

The issue has become a flash point on both sides of the Atlantic. A Washington-based anti-abortion group, Americans United for Life, said on Thursday that its president was “in London helping on the Charlie Gard case,” and that the boy “deserves a chance at life.” Some demonstrators protested outside the court. The judge warned that the hospital had received threats.

The parents’ lawyer, Grant Armstrong, insisted that the case “bristles with differences of medical opinion,” despite the consensus of the British doctors.

Experts have deplored the politicization of the case.

“Only the family, the doctors treating Charlie, and now the legal teams involved, know the details of complex issues that define his situation,” Neena Modi, president of the Royal College of Paediatrics and Child Health, wrote in an open letter on Monday.