Specialist cancer nurses are being "run ragged", with high vacancy rates meaning patients may not be receiving the level of specialist care they need, a report has suggested.
Macmillan Cancer Support said there is a "startlingly broad variation" in the number of specialist cancer nurses in some parts of England compared to how many new patients are being diagnosed.
It said numbers of new lung cancer cases each year per specialist nurse vary from 62 to 203 in different areas, while for breast cancer nurses they vary from 56 to 145.
High vacancy rates also mean that as many as one in seven chemotherapy nurse positions are unfilled in some places.
It said its census of specialist cancer nurses and support workers has thrown up some major challenges which threaten the future of cancer nursing, warning some patients may not be getting access to much-needed specialist care.
The charity said the census - the most detailed research to date on this part of the NHS's workforce - looked at four roles - specialist cancer nurses, chemotherapy specialist nurses, specialist palliative care nurses, and cancer support workers.
It found vacancy rates higher than the UK rate for health and social work across all four roles.
The charity said the report highlights a "worrying trend" of specialist cancer nurses being paid in lower pay bands than in 2014, when such research was last carried out.
It also warns that the proportion of specialist cancer nurses aged 50 or older continues to climb.
This is happening as the number of people with cancer is increasing - with more than 830 cancers diagnosed every day in England.
Dr Karen Roberts, the charity's chief nursing officer, said: "Having the expertise and support of a specialist nurse from the point of diagnosis has a huge bearing on whether or not a cancer patient has a positive experience of the care they receive.
"We are concerned that cancer nurses are being run ragged, and that some patients may not be receiving the level of specialist care they need.
"Nurses working in cancer care tell us that their increasingly complex and pressured workload is beginning to affect the quality of care patients receive. It is no surprise that hospitals are struggling to recruit to these roles, given this unprecedented pressure."
Its executive director of policy, Dr Fran Woodard, said: "While the cancer workforce has grown, it has done so over a number of years without adequate long-term planning or direction.
"Macmillan has undertaken this work to highlight the strain this puts on those working in cancer care and to ensure that action is taken. This situation will become more acute as the number of people being diagnosed with cancer continues to grow.
"We welcome the progress Health Education England (HEE) is making on the cancer workforce strategy. However, this census highlights the urgent need for this essential part of the NHS workforce to be properly equipped to cope with the increasingly complex challenge that cancer poses in the years to come, and it is therefore vital that the Department of Health and Social Care ensures that the cancer workforce strategy is appropriately funded".
Ann McMahon, research and innovation manager at the Royal College of Nursing, said it was no exaggeration to say the shortage of nurses is "putting patients' lives at risk".
"For patients diagnosed with cancer, any delay can lead to worse outcomes, and it is difficult to overstate the distress felt by patients and their families as they wait to begin the treatments lives can depend on," she added.
"The blame for this rests solely with the Government. Poor workforce planning and brutal cuts to training budgets have left specialist services struggling to recruit the skilled nurses they need. Patients deserve better than this, and ministers must look again at the recruitment, training and retention of specialist nursing staff."