Margaret Lowbridge, 74, of Oldbury, says she was told by Professor David Ferry, her consultant at Dudley's Russells Hall Hospital, to go home and arrange palliative care.
She says she only learned of the error when she was sent a letter asking her to attend a follow-up appointment 18 months later.
Mrs Lowbridge has now developed lung cancer, which she believes may be because she was advised not to undergo any more treatment, causing her existing cancer to spread.
She was first diagnosed with bowel cancer in January 2009 and underwent surgery. She was left with a colostomy bag and had chemotherapy.
The treatment seemed to have been a success until June 2013, when a regular check-up showed a lump.
Mrs Lowbridge said she was referred back to Professor Ferry who offered her chemotherapy but the treatment caused a deep tissue infection and was withdrawn.
She says she was then told no further treatment could be offered and she should contact a Macmillan nurse in relation to palliative care. She said she was also told she would not be alive in six months and should arrange her funeral.
"I was devastated by the news, as was my daughter," Mrs Lowbridge said. "I was left counting down the months until my anticipated death which never arrived. But I did as advised and the Macmillan team helped me with pain relief."
Law Firm Medical Accident Group, which is now representing Mrs Lowbridge, said she had been surprised when a letter had come from Russells Hall Hospital last December inviting her to attend a follow-up appointment. She only attended because her daughter persuaded her to.
A legal claim for damages is now being investigated by Medical Accident Group.
Professor Ferry is currently working for a US drugs firm.
Chief executive of the Dudley Group NHS Foundation Trust Paula Clark said: "The trust cannot comment on the treatment of Margaret Lowbridge for reasons of patient confidentiality.
"Professor Ferry was employed by the Royal Wolverhampton Hospitals NHS Trust, not the Dudley Group."
The Royal Wolverhampton Hospitals Trust said it could not comment for legal reasons.
The Medical Defence Union, which provides legal cover for doctors including Prof Ferry, said he is 'unable to comment due to his duty of confidentiality to his patient'.