Theresa May has said there are "absolutely no plans" for a hospital trust to reduce or delay the start of chemotherapy treatment for cancer patients.
The Prime Minister was pressed on reports from a leaked memo circulated to staff at Churchill Hospital in Oxford.
According to The Times, the memo warned the number of chemotherapy cycles offered to the terminally ill would have to be cut because of a lack of staff trained to deal with medication.
At Prime Minister's Questions, former shadow health minister Luciana Berger questioned if Mrs May would apologise for the "appalling situation".
The Prime Minister said: "The trust has made clear there are absolutely no plans to delay the start of chemotherapy treatment or reduce the number of cycles of treatment given to cancer patients.
"Again, what (NHS England chief) Simon Stevens has said is happening in the NHS in relation to this is, over the past three years, highest cancer survival rates ever.
"Latest survival figures show an estimated more than 7,000 more people surviving cancer after successful NHS cancer treatment compared to three years prior.
"And we do see more, 3,200 more diagnostic and therapeutic radiographers, than in May 2010.
"So we will continue to look at this, but we're continuing to put the funding in that is enabling us to improve our treatment for cancer patients."
Oxford University Hospitals NHS Foundation Trust said in a statement it had made no change to its formal policy for chemotherapy treatment.
Labour MP Ms Berger said: "The Health Secretary said that the Government wanted to be the best in the world for cancer diagnosis, treatment and care.
"Today, according to a memo from the head of chemotherapy at Oxford Churchill Hospital, terminally ill cancer patients will have their chemotherapy cut because of a massive shortfall in specialist nurses.
"Will the Prime Minister apologise to cancer patients and their families for this appalling situation?"