Detecting cancer at an earlier stage could save between 5,000 and 10,000 lives in England, according to research.
The government's cancer services chief Professor Mike Richards said up to 10,000 people die of the disease every year because their condition is diagnosed too late.
The National Cancer Director branded the current detection situation as "unacceptable".
He said: "These delays in the patient presenting with symptoms and cancer being diagnosed at a late stage inevitably costs lives. The situation is unacceptable so the first big step has been to understand why the delays occur."
His findings - which come amid growing concern over the quality of healthcare in the NHS - are to appear in an article in the forthcoming British Journal of Cancer.
Prof Richards' conclusions came after a study of Britain's three biggest cancer killers: lung, which killed 34,589 people in 2007; colon which claimed 16,087 lives; and breast cancer, which killed 12,082 people.
They comprised about half of all the deaths that could have been avoided if diagnosis was as good as the best-performing European countries, he said.
Prof Richards' staff are working with GPs to carry out a national audit revealing the extent of delays and where they exist.