In world first cases, NHS doctors have completed heart transplants in children with the organs brought back to life using a pioneering machine.
The new technique has saved the lives of six British youngsters aged from 12 to 16, and the transplants have all happened during the coronavirus pandemic.
Donated hearts have historically been from people who are brain dead but their hearts are still beating which limits the scope for how many transplants are possible.
And on average, children have to wait two-and-a-half times longer than adults for hearts to become available.
Now Royal Papworth Hospital medics in Cambridgeshire have made hearts start to beat again after they have stopped, and the organs have been successfully transplanted into youngsters at London's Great Ormond Street Hospital.
The hearts were brought back to life using a machine called an organ care system (OCS).
After death, the donor heart is retrieved and put on the OCS, where it is kept warm and has 1.5 litres of the donor's blood pumped through it in a cycle, and receives nutrients.
Doctors can also speed up the heart rate by remote control if needed.
The hearts were then flown to London for transplanting.
The technology had previously been tried only in adults.
The first child whose life was saved by the heart-in-a-box machine and a stranger was Anna Hadley, now 16, from Worcester.
She had waited almost two years for her heart transplant after being told she had a terminal condition.
Within 24 hours of the five-hour operation in February last year she was sitting up in bed. A fortnight later, she walked out of Great Ormond Street and she was playing hockey within weeks.
"I just feel normal again. There's nothing I cannot do now," she told The Sunday Times.
Another patient, 14-year-old Freya Heddington from Bristol, said: "This new procedure has prolonged my life, it's saved my life and it's given me new opportunities that I didn't have beforehand."
"No one else in the world is currently doing this," said Marius Berman, consultant cardiothoracic transplant surgeon at Royal Papworth.
Dr John Forsythe, medical director for organ donation and transplantation at NHS Blood and Transplant, said: "This new technique will save lives both here and around the world.
"It means people can donate their hearts where it wouldn't have been possible in the past, giving life to patients on the waiting list."
The breakthrough is expected to allow a big expansion in the number of donor hearts available, reduce post-operation complications, speed recoveries, increase transplant survival rates and save hundreds of lives.