Health Secretary Alan Johnson has defended a health authority for refusing to pay for care if a cancer victim pays for extra drugs.
Mrs Mills is faced with a £1,500-a-month bill
Colette Mills, 58, from North Yorkshire, is being treated with Taxol for breast cancer but believes her chances would be improved with Avastin.
Ms Mills says she would fund the drug but South Tees NHS Trust says if she did it would stop paying for her care.
Mr Johnson said patients must be treated either by the NHS or privately.
Speaking in the Commons on Tuesday he said: "That way lies the end of the founding principles of the NHS."
If Ms Mills, from Hutton Rudby, had to pay for her own care it would cost an estimated £15,000 a month.
South Tees Hospitals NHS Trust said in a statement: "If a patient chooses to go private for a certain drug they elect to become a private patient for the course of their treatment for that condition.
"That is trust policy."