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The children and their immediate family members landed on Sunday evening from Poland and will be triaged before being sent to NHS hospitals to continue their care.
The youngsters were taken to England by the UK government with the support of Polish authorities and doctors, the department said.
Their cancer treatment will be provided free of charge by the health service across hospitals in England.
Hospitals in Poland have taken in children needing treatment who have arrived from Ukraine, the department said.
It said that with more children crossing the border requiring immediate treatment, the UK has “responded to Poland’s call for support from international partners to provide additional care”.
The UK formed a partnership with St Jude Children’s Research Hospital, a non-profit American organisation that specialises in paediatric diseases, to arrange an urgent flight for the children.
"The sickening suffering of innocent civilians in Ukraine is truly horrific," said British Prime Minister Boris Johnson.
“This vital lifesaving medical care is another important step in our support for the people of Ukraine and their resistance against Putin’s illegal invasion.
“I am hugely grateful to our fantastic NHS staff as well as our partners, including our Polish friends, for their support in bringing these children to the UK and we will continue to do all we can to support them as they continue their critical treatment here.”