Hospitals have had to cancel surgery for patients suffering from cancer and heart problems as NHS trusts struggle with a lack of beds.
NHS England had advised hospitals to defer non-urgent operations until the end of January in a bid to free up beds and hospital staff amid winter pressures - but trusts have said they had to cancel cardiac procedures and cancer operations too.
Delaying surgery for someone with a life-threatening condition increases risk and is a "nightmare" for the patient, the Patients Association said.
Oxford University Hospitals NHS Foundation Trust said that 16 cardiac surgery operations were postponed on the day of surgery or on the day before surgery because of a lack of ICU beds during December and January.
During the same time period, the trust said eight cancer operations were postponed on the day of surgery or on the day before surgery because of a lack of ICU beds and a lack of beds on inpatient wards.
A spokesman for Oxford University Hospitals NHS Foundation Trust said: "We recognise that cancelling operations at short notice, either on the day of surgery or on the day before surgery is due to go ahead, can cause great anxiety to patients and their families.
"We would like to apologise for any distress caused to patients who have been affected.
"Cancelling operations at short notice is usually due to a lack of critical care beds or beds on inpatient wards because of the high numbers of emergency patients being treated in our hospitals this winter."
University Hospitals Birmingham said it had cancelled 34 cancer operations, 53 cardiac procedures and four aortic aneurysm repairs throughout December and January.
A spokeswoman said there were 167 cardiac procedures listed over the time period, and 53 were cancelled which involved 40 patients.
Of those 40 patients, 16 had their procedure within 48 hours.
The spokeswoman confirmed that the number of cancellations was higher than normal.
University Hospitals of North Midlands medical director Dr John Oxtoby said: "Due to severe and sustained pressure on our services in the North Midlands this winter we had to prioritise some urgent surgery.
"There has been a substantial increase in the number of admissions into our intensive care unit and we cannot perform major surgery without a critical care bed being available post surgery.
"This includes 635 patients who have tested positive and been admitted for flu since the start of November 2017, of which 95 have needed a critical care bed."
Rachel Power, chief executive of the Patients Association, said: "The cancellation of an operation for someone with a life-threatening condition is always deeply undesirable, but there is always a risk of it - crucial team members can fall ill, for instance.
"The challenge for the NHS is to have enough resilience to minimise these cancellations as far as possible.
"We presume that these cancellations are separate from the decision to cancel elective surgery during January, but would like to know from NHS England whether cancellations of this sort have been happening at higher rates during this winter than at other times.
"Delaying surgery in these instances increases the risk to life and is a nightmare for any patient subjected to it."
An NHS England spokesman said: "The National Emergency Pressures Panel asked hospitals to defer non-urgent operations to free up capacity to deal with winter pressures.
"However, decisions on whether or not to go ahead with a procedure are rightly always made locally by clinicians with the best interests of the patient in mind."