Thousands of people diagnosed with cancer may be being failed by GPs who are not referring them to specialists quickly enough, contravening official targets.
The figures from around 4,000 GP practices in England show that, in many cases, only a minority of patients are fast-tracked for investigation by a specialist.
Although NHS targets stipulate 95% of patients with suspected cancer referred by their GP must be seen by a specialist within two weeks, data from NHS England says many are not seen on this basis and are eventually diagnosed another way.
In some practices, only around one in 10 patients eventually diagnosed with the disease saw a specialist within two weeks.
Mike Bewick, deputy medical director at NHS England, said the level of variation between practices is too wide, adding: "When people go to their GP with red flag symptoms such as coughing up blood or changes in their bowel you would expect those patients to be picked up."
"But just imagine the scenario when you have an elderly patient with many symptoms and the one they are most worried about is not the red flag.
"It's often due to complexity rather than mistake."
While some GP practices show 100% of patients with cancer making it through the fast-track system, others fall far behind.
In about half of the practices in the sample, fewer than 50% of cancer patients were seen through the two-week system.
Not all patients with cancer visit their GP with symptoms.
Some are diagnosed in A&E, while others have cancer detected during routine tests, or are referred straight to A&E by their GPs because symptoms are so bad.
The health secretary, Jeremy Hunt, said: "Every single patient in the NHS has a right to the very best care – and to see a GP who can spot cancer symptoms early enough to make a difference.
"That's why we've introduced a rigorous new inspection regime for GP surgeries to tackle this unacceptable variation across the country.
"The new chief inspector will speak up for patients without fear or favour, rating each surgery so we can celebrate the best practices and take tough action where standards aren't up to scratch.
"GP surgeries are the bedrock of our NHS, providing vital long-term care for some of our most vulnerable and elderly citizens. I am determined that patients who rely on them get the excellent service they deserve every time."
Stuart Barber, head of communications and campaigns at Beating Bowel Cancer, said: "GPs have the tools. There are clear symptoms, there is a clear screening programme and if a patient visits their doctor with what are symptoms of bowel cancer they should have the confidence they are going to be referred quickly.
"It's intolerable that patients are having to wait. We know of cases where patients have gone multiple times backwards and forwards to their GP with what they think are bowel cancer symptoms. The symptoms get worse and worse and worse and it does turn out to be cancer.
"We know of patients who have seen their GP two or three times over a period of a year, year and a half.
"Some patients are being told it's IBS (irritable bowel syndrome) or another condition."
A report last year by the Welsh government found the NHS is failing to meet the target stating that at least 95%of patients should start cancer treatment within 62 days of being referred by their GPs.
It found that the 95% figure had not been met at an all-Wales level since 2008.
