Almost a quarter of people are not diagnosed with cancer until they are admitted to hospital as an urgent case, research suggests.
The National Cancer Intelligence Network (NCIN) said 24% of people only learn they have the disease in accident and emergency wards.
Research from the NCIN also shows the figure is even higher among people over the age of 70 - 31%.
Cancer charities say such late diagnosis can have a "disastrous impact" on survival chances.
The research, published in the British Journal of Cancer, examined more than 730,000 cancer patients between 2006 and 2008 in England.
Sara Hiom, director of information at Cancer Research UK and one of the study authors, said: "Our findings showing the sheer numbers of cancer patients first seen as an emergency are startling.
"Early diagnosis of cancer, when the most effective treatments are more likely to be options, helps improve a patient's chance of surviving their disease.
"We don't yet know the reasons that lie behind these stark figures but, although we might expect higher numbers of older patients to have cancer detected as an emergency, we urgently need to understand why there is such a great proportion."
Professor Jane Maher, chief medical officer at Macmillan Cancer Support , added: "This route to diagnosis can have a disastrous impact on survival chances.
"It can be more difficult to spot cancer symptoms in older people who have other health conditions but this does not excuse such a high number of people being diagnosed in this way.
"All cancer patients should be given the best possible survival chance and we owe it to the older members of our society to ensure that this applies equally to them."