Comedian Al Murray is urging people to sign the blood stem cell register after his nephew was struck by leukaemia.
Six-year-old Finley Relf has a rare and aggressive form of the disease and his only hope of a cure is a bone marrow transplant.
Finley’s parents Ben and Nicky Relf sought medical help after noticing that their son was losing weight.
He was diagnosed with juvenile myelomonocytic leukaemia (JMML), a rare type of blood disorder that occurs in young children.
The youngster, from Haywards Heath, West Sussex, is having chemotherapy at London’s Great Ormond Street Hospital.
Murray’s partner Eleanor Relf is Ben Relf’s sister.
Murray told the BBC’s Victoria Derbyshire programme: “Finley is undergoing chemotherapy but his best chance of fighting the disease is a bone marrow transplant.
“He has JMML, a super-rare leukaemia and 12 kids a year get it probably, and he needs a blood stem cell transplant.”
Murray said Finley was undergoing “gruelling” treatment but was a “chirpy six-year-old who wants to run around and go swimming”.
“He’s critically ill, there is a 50/50 chance of him surviving this basically, and without the transplant, there is no cure for JMML.”
Murray said Finley needs a transplant “before Christmas, this has got to happen right away”.
Jonathan Pearce, chief executive of the blood stem cell charity DKMS, which is looking for a donor for Finley, said: “Even though we are working with Al Murray in very sad and serious circumstances, he is doing fantastic work with us to highlight that only 2% of the UK population are on the blood stem registry.
“We desperately need more people to come forward as donors if we’re to find a match for every person with blood cancer.
“Blood cancer is the third most common cause of cancer death in the UK, and at any one time there are around 2,000 people in the UK in need of a blood stem cell transplant.
“I would urge anyone between 17 and 55 and in general good health to request a swab kit from dkms.org.uk so we can help find a match for Al’s nephew Finley and all the other people that need one.”