Twelve-year-old Fahim, who was featured in yesterday's Independent, is being treated at the Royal Free Hospital in Hampstead, north-west London, after suffering a relapse.
He has had two courses of chemotherapy since arriving in the country from Tanzania last June, but now needs a bone marrow transplant.
He is not eligible for NHS treatment and his parents, Amin and Nasim, have raised pounds 35,000 through appeals in the Independent and Harrow Observer, together with pounds 10,000 the family brought with them, to pay for it.
The Home Office has ruled that Fahim can stay with his aunt, who lives in Hillingdon, west London, but the rest of the family must return to Tanzania.
Members of the family including Fahim's 16-year-old brother, were admitted as medical visitors last June on the understanding that they could support themselves.
When they applied for an extension last December, Mr and Mrs Manji sought permission to work because funds were running out.
This appears to have triggered the order for them to leave. The Home Office said it would consider an appeal for one of them to stay, provided they could prove they had sufficient funds.
Mr Manji said: "We were told Fahim's treatment could last two years and we can't go on depending on relatives. We have a 12-year-old boy with leukaemia soon to have a transplant. If I go back to Tanzania I am not going to get any sleep. It is terrible."
Parvinder Bhogal, the family's solicitor, described the decision as "harsh".
"The Home Office can make an exception to the immigration rules but they have decided this is not an exceptional case. They would have allowed the family to stay if they could have supported themselves but they won't let them work so that they can do so. They are in a Catch- 22."
t Donations for Fahim's medical treatment may be sent to the Fahim Manji Appeal, 326 Station Road, Harrow, Middlesex, HA1 2DR.