Thousands of people diagnosed with cancer in A&E every year have visited their GP three times or more with symptoms, research shows.
The most comprehensive study to date found that 71% of all patients diagnosed as an emergency had seen their GP at least once with symptoms that turned out to be cancer. The remainder had never visited their GP.
Of the group that did see their GP with symptoms, 41% had sought help three or more times - while 59% had seen their GP once or twice.
Some of these had difficult-to-spot cancers, such as lung cancer or multiple myeloma, and tended to be younger or female.
But the group also included people with common cancers such as breast cancer (31% of breast cancer patients had visited their GP three or more times), bowel cancer (41% had visited three or more times) and prostate cancer (37% had visited three or more times).
People who are diagnosed with cancer as an emergency have a worse prognosis than those diagnosed at an earlier stage.
Cancer Research UK data shows that 22% of people diagnosed with cancer each year are an emergency case.
This suggests that of the more than 356,000 people diagnosed with cancer each year in the UK, over 78,000 are diagnosed as an emergency.
In the new study, published in the British Journal of General Practice (BJGP), the authors - including from University College London, the University of Cambridge and Public Health England (PHE) - analysed 2010 data from 4,637 people diagnosed in A&E.
They found those patients who had never been to their GP tended to be older, male and living in the most deprived regions of England.
Meanwhile, patients in A&E diagnosed with common cancers, such as breast cancer, who had visited their GP three times or more, may be presenting with atypical symptoms, they said.
Dr Georgios Lyratzopoulos, one of the lead researchers based at UCL, said: "These findings tell us that some patients diagnosed as an emergency might not be acting on 'red flag' symptoms which could have prompted them to visit their GP.
"There's also a host of other factors that may be at play. For example, many elderly patients may find it difficult to get to the surgery or have other conditions which would prevent them from seeking an appointment, such as dementia."
Samia al Qadhi, chief executive of Breast Cancer Care, said the findings were "extremely concerning", adding: "We speak to women and men who tell us how incredibly distressing it is not to be taken seriously when they've found a possible symptom.
"We have found almost a third (31%) of people with incurable secondary breast cancer do not feel healthcare professionals listened to their concerns about having the disease before being diagnosed.
"And nearly a 10th (8%) are diagnosed in A&E rather than a breast clinic, indicating 'red flag' symptoms for incurable breast cancer are not being acted on appropriately."
Judith Brodie, acting chief executive of Beating Bowel Cancer, said: "It's concerning that the study shows 41% of bowel cancer patients who are diagnosed as emergencies had previously sought help from their GP three or more times.
"A bowel cancer patient's chance of being successfully treated drops dramatically if they are not diagnosed until a late stage, so more must be done to ensure that the public is aware of the symptoms and how important it is to get them checked out as soon as possible.
"Knowledge of the disease will also give them the confidence to persevere with their GP if they feel their symptoms are not being taken seriously enough."
Dr Julie Sharp, head of health information at Cancer Research UK - which funded the study, said there were multiple reasons that affect how and when a cancer diagnosis is made.
"We need to continue to increase awareness of cancer signs and symptoms and help break down the barriers preventing people from seeing their GP earlier, whilst GPs need better access to the right tests and referral routes if we want to see this number reduced," she said.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said the proportion of cancers diagnosed as an emergency is dropping and more patients are being diagnosed at an earlier stage.
"However, as this study shows, there are still some patients who seem to be missing or ignoring worrying symptoms until they are severe enough to send them to A&E.
"GPs take their role in diagnosing cancer as early as possible very seriously and we would urge patients who experience any concerning or persistent symptoms to book an appointment with their GP."