Children with some forms of cancer in Leicestershire will have to travel to Nottingham for treatment following the results of a service review.
Treatment for solid tumours will move from the Leicester Royal Infirmary (LRI) to clinics at Nottingham's Queen's Medical Centre.
Roz Barnes, whose son has brain cancer, said the trip might be "very dangerous" for children with infections.
Hilary Killer from the LRI said medical expertise could now be pooled.
"Over the past two years we have looked at developing high quality, sustainable services for the future," said Ms Killer.
"This is not about reducing services in Leicester, it's about pooling expertise.
"Parents have been involved throughout this process and the clinicians have guided the process. We have to listen to the experts and they are saying this is the best for children to ensure the best outcome - any parent would want that."
But Mrs Barnes from Fleckney, whose four-year-old son Alex is recovering after pioneering treatment in the US, said the move was disastrous.
"For those parents whose children are on treatments and have to be there [at the hospital] every day, it is going to be a disaster.
"It's not easy to get from Leicester to Nottingham and it's time-consuming. It is very dangerous for children who have cancer because they can get very dangerous infections straight away and an hour or more in the car can cause a lot of problems and can be life threatening."
Chairman of the Leicester Patients' Group, Zuffar Haq, echoed Mrs Barnes' concerns. "Just imagine if you've got to get a bus from Fleckney or Market Harborough to go all the way to Nottingham, it's going to be a full morning's trek."
Ms Killer said the decision would be under constant review.
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