Until now, patients have been excluded from the NHS if they pay for treatment not available on the health service, but in practice this varies from place to place.
Mr Johnson has announced that he will lift the ban, following a four-month review of the issue by national clinical director for cancer Professor Mike Richards.
However, the government has stated that any private treatment should take place in a private facility and must not be subsidised by the NHS.
'A small number of patients may still choose to pay for additional drugs not available on the NHS. But I have agreed that, from today, NHS care must never be withdrawn in these cases-as long as private treatment takes place in a private facility'.
The Department of Health announced the review in June, after an outcry from patients over what was considered a hard-line stance and increasing evidence of an inconsistent approach by individual NHS trusts.
In total, 15,000 patients appealed to their local NHS trust last year after being denied treatment although according to Professor Richards up to 75 per cent patients who appealed were successful.