Cancer patients could be at risk due to overstretched and "exhausted" health staff in a "system at breaking point", England's NHS ombudsman has warned.
Rob Behrens' office carried out 1,019 investigations related to cancer between April 2020 and December 2023 - with 185 upheld or partly upheld.
Issues included delays in diagnosis, treatment and pain management.
The Department of Health and Social Care said record numbers of people had been treated in the last two years.
Meanwhile recent analysis by the OceanNewsUK revealed cancer waiting times in England last year were the worst on record.
Mr Behrens, the parliamentary and health service ombudsman who investigates unresolved complaints against NHS England, said everyone deserves safe and effective care.
He added: "But patient safety will always be at risk in environments that are understaffed and where staff are exhausted and under unsustainable pressure."
He called for concerted and sustained government action to ensure NHS leaders could prioritise the safety of patients.
"A key part of this is investing in the workforce, for today and for the long term, including providing full funding for the long-term workforce plan," he added.
Problems investigated by his team included concerns about poor communication, end-of-life care and how complaints were handled.
Most were about lung cancer, followed by breast cancer and colorectal cancer - some of the most common cancers in the UK.
One investigation involved the death of Mrs Sandra Eastwood, whose rare cancer was not diagnosed for nearly a year, after scans were not interpreted correctly.
The delay meant she missed out on the chance of very effective treatment that could have prolonged her life, according to the report.
Mrs Eastwood, who lived in York, died in 2022, with a rare digestive system cancer called gastrointestinal stromal tumour.
In June 2020 she went to hospital with abdominal pain.
CT scans showed a mass which medics put down to a blood clot caused by medication she was taking for a unrelated heart valve replacement.
The following May her symptoms worsened and she returned to hospital and was diagnosed with GIST.
The report found that if Mrs Eastwood had been diagnosed earlier and scans interpreted correctly, the cancer might not have spread and she may have been eligible for surgery.
John, her husband of 54 years, said: "Sandra was wonderful. I worked away a lot when our two children were young and she did absolutely everything for them. She loved baking, making jams and chutneys, and travelling.
The 79-year-old continued: "I feel absolutely disgusted with the care she received from the hospital. They didn't investigate the scan results and just put it down to Warfarin [her medication] straight away.
"It seemed like the medical teams did not communicate with each other and everything felt very disjointed. They left her in agony for months before she died.
"The whole experience was very distressing, which is why I went to the ombudsman. I didn't want this to happen to anybody else."
A spokesperson for York and Scarborough Teaching Hospitals NHS Foundation Trust said it "fully accepts" the ombudsman's findings and apologised for the distress caused to Mrs Eastwood and her family.
The spokesperson added that the trust has made a number of improvements and agreed to change its practice following the case.
Mr Behrens said: "What happened to Mrs Eastwood was unacceptable and her family's grief will no doubt have been compounded by knowing that mistakes were made in her care.
"Her case also shows, in the most tragic of ways, that while some progress has been made on my recommendations to improve imaging services, it is not enough and more must be done.
"Government must act now to prioritise this issue and protect more patients from harm."
The Department of Health said: "The NHS has seen and treated record numbers of cancer patients over the last two years and cancer is being diagnosed at an earlier stage, more often, with survival rates improving across almost all types.
"We have invested £2.3bn into speeding up diagnosis and launched 154 community diagnostic centres across England.
"The NHS long-term workforce plan will boost the number of healthcare workers diagnosing and treating cancer, doubling the number of medical school and adult nurse training places."
