More than 20,000 people have signed a petition demanding a Nigerian woman be allowed into the UK to donate bone marrow to her sister who has leukaemia.
May Brown, 23, from Weymouth in Dorset, said she had been told by her consultant that her sister Martha was a “10 out of 10” tissue match.
The African Caribbean Leukaemia Trust (ACLT) said Martha had been refused a visa because her income was too low.
The Home Office said immigration rules were applied to all visa applications.
Karl Blick, from Smethwick, in the West Midlands, said he had signed the petition “because human lives are more important than borders and petty rules”.
“I’m ashamed of my country, ” said Bridget Chapman, from London, adding: “This is cruel and inhuman. Fix it now!”
Mrs Brown, a patient at King’s College Hospital in London, has been told her only chance of survival is an urgent stem cell transplant.
Beverley De-Gale, co-founder of ACLT, said Mrs Brown started chemotherapy on Friday in the hope the surgery would go ahead.
“The hospital has no choice but to commence with chemo so the transplant can take place in three weeks,” she said.
“We have no idea whether Martha will be here by then, but we are hoping common sense prevails.
“Martha will need to go through a medical assessment before the donation itself can take place.”