The Liberal Democrat health spokesman Dr Evan Harris is stepping down from his post, reportedly to care for his terminally-ill girlfriend.
Dr Harris said the problem was the NHS's "doctor knows best" policy
Dr Harris, 37, is said to want to care for 29-year-old girlfriend Liz O'Hara, who is suffering from a malignant brain tumour.
The Daily Mail said he is angry Miss O'Hara's tumour was not diagnosed properly last year. The paper quotes him as saying she was twice sent away by her GP with headache pills after suffering from headaches last May.
When she was later sent to hospital Dr Harris said his plea for a brain scan was ignored by a junior doctor.
"I am angry that in this case, despite there being clearly detectable symptoms and signs of something very wrong, that she was sent home from casualty," the MP for Oxford West and Abingdon told the newspaper.
He has refused to name the NHS health trust involved.
"I'm not generalising about the NHS but I am saying there is a lesson to be drawn from this case," the newspaper reports.
Dr Harris, who is a qualified hospital doctor, said the misdiagnosis was an example of the "doctor knows best" culture in the NHS.
The newspaper says Miss O'Hara's tumour was only diagnosed when she returned to her native Ireland.
She is suffering from glioblastoma multiforme, a malignant form of brain tumour.
Miss O'Hara's symptoms included headaches, vomiting, and a change in personality.
Although doctors in Dublin removed the tumour after Miss O'Hara had lapsed into a coma, her condition is still terminal, the Daily Mail says.
"Diagnosis could and should have been made earlier and the reason for the omission was a straight forward failure to take seriously the complaint of the patient and the concerns of her family," Dr Harris is quoted as saying.
"Hospital doctors, particularly the junior ones who work at night, are
working under great pressure but in this case the diagnosis should have been made."
Dr Harris said his partner was "doing well" and the couple had been travelling the world, visiting Egypt, Prague and South Africa.