Senior doctors say they are concerned about the number of cancer patients having their operations cancelled amid “tremendous pressure” facing the NHS in England this winter.
Ian Eardley, from the Royal College of Surgeons, said a shortage of beds was one of the causes.
Such surgery is usually protected under treatment time guidelines, he added.
The NHS said it was “pulling out all the stops” to ensure patients receive surgery “as quickly as possible”.
A spokesman for NHS England said there had been a steady increase in the number of operations over the last 15 years and more people were surviving cancer than ever before.
Guidelines for hospitals in England stipulate that cancer patients should be seen within 31 days and receive primary treatment within 62.
In November 2016, the latest period which NHS figures are available for, the 62-day target for treatment to start was missed – with 83.5% of patients being treated in that timeframe instead of 85%.
Mr Eardley, vice president of the RCS, said most hospitals were able to see more than 90% of patients within that time period, but in the past year “it’s been more difficult to achieve that”.
He said that while cancer operations were cancelled “from time to time”, especially during the winter, the RCS had heard from its members in England about an increasing number of cancellations within the last week.
“There are current pressures – since Christmas particularly – and the number of cancelled operations has been going up,” he told the BBC.
“The NHS is under tremendous pressure – more and more patients are going to A&E and there is more difficulty in getting patients home, and it’s not something we are comfortable with at all.
“If we could get patients home more quickly and effectively, we could carry on with doing surgery more quickly and more effectively.”
He added that solving the shortage of beds caused by problems with arranging care in the community would be “the easiest thing to do most quickly, although there are other longer-term problems and there also needs to be a broader review of the NHS.”
Analysis – by Hugh Pym, BBC News health editor
It’s a graphic illustration of what the current extreme pressure on the NHS means for patients.
Routine surgery is often cancelled during the winter to allow hospitals to keep beds free for the expected surge in emergency admissions, but cancer treatment always continues with very few exceptions.
In what looks like a highly unusual move, several hospitals are now having to postpone cancer operations because of a shortage of beds.
This comes after news that more than four in ten hospitals in England were on major alert status because of high patient numbers in the week ending 8 January.
All this is at a time when the weather is mild and there has not been a major flu outbreak.
If that changes, the NHS will come under even greater strain.
Richard Murray, director of policy at The King’s Fund think tank, said the cancellations “reflect capacity problems in the NHS”.
“There are not enough beds, there are more patients arriving and hospitals are not able to discharge them quickly, particularly for specialisms like orthopaedics,” he said.
“You should never cancel operations on the day for non-clinical reasons, but especially for cancer treatment. It causes distress for patients and it’s an urgent pathway. Critically, it becomes how quickly you can rebook.”
A Freedom of Information request from the BBC’s Inside Out programme suggested that across the UK, the number of cancer operations cancelled on the day of surgery rose from 403 in 2011-12 to 810 in 2015-16.
However, only 95 of 205 health boards and trusts in the UK responded to the FOI request, and a figure for the total number of cancer operations performed was not available.
Lynne Roper, from Devon, who died in August 2016, had her surgery to remove her brain tumour at Derriford Hospital cancelled at the last minute earlier in the year.
Her mother Jenny Roper told Inside Out: “It was really traumatic, I cried. We got to the hospital terribly early, when we left she was gowned ready to go and then when we got home she called and said ‘they haven’t got a bed for me’. She was extremely upset.
“We don’t have a ‘beef’ with the health service or Derriford, but we do with the government cutbacks. We were all pretty devastated, the whole time there was a tumour growing in our daughter’s head.”
Plymouth Hospitals NHS Trust pointed out it was a main centre for cancer treatment in the region and said: “Cancelling a patient’s operation is not a decision that is ever taken lightly; it is often the end result of a difficult balancing act we are faced with.
“We understand the distress and inconvenience a cancellation can cause our patients and their families and for this we are extremely sorry.”
Philip Rolle, 52, from Redditch, had his bowel cancer surgery cancelled twice at Birmingham’s Queen Elizabeth Hospital last year.
He said: “The hospital was full, they didn’t know where the next bed was coming from. It was very frustrating for everyone, all my friends were shocked and appalled.
“When it happened a second time I was alarmed because the cancer will spread.”
University Hospitals Birmingham NHS Foundation Trust said: “This cancellation was a reflection of an unprecedented 6% rise in attendances to our emergency department which increases the pressure for beds within the hospital – including intensive care beds.”
Andrew Claridge has prostate cancer and said he was told his operation had been cancelled one day before it was due to take place.
“You’ve got cancer inside you, you just really want to get rid of it. It’s just devastating to get that type of news,” he said.
Mr Claridge said it has been very upsetting for his family, but he has sympathy for his doctors and he received an apologetic phone call from an NHS consultant.
“I feel sorry for them, I really do, because they don’t want to give bad news – they want to go in there and say, ‘Mr Claridge, you’ve had your operation, it’s all been successful’.
“But to tell somebody that you can’t have your operation – that’s life threatening, whether I’ve got ‘x’ number of days or whether it’s longer – it’s still scary,” he said.
“How long have I got before I’m not going to have the chance to have an operation to get this removed?”
Dr Moira Fraser, director of policy and public affairs at Macmillan Cancer Support said: “Nobody should have to go through the distress of making emotional and practical preparations for an operation only to have it cancelled.”
BBC Inside Out – NHS Special will be shown on BBC One on Monday 16 January at 19:30 GMT.