Thousands of people eventually diagnosed with cancer may be failed by GPs who do not refer them quickly enough, data suggests.
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.
In some practices, only around one in 10 patients eventually diagnosed with the disease saw a specialist within two weeks.
The target for the NHS says 95% of patients with suspected cancer referred by their GP must be seen by a specialist within two weeks.
Today's data suggests many are not seen on this basis and are eventually diagnosed another way.
While some GP practices show 100% of patients with cancer making it through the fast-track system, others fall far behind.
In around 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 their symptoms are so bad.
The new data has been published by NHS England as part of a raft of information to help patients work out how well their GP practice is performing.
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."
Mike Bewick, deputy medical director at NHS England, said the level of variation between practices is too wide.
He said the data offered an "important insight for commissioners as to where we should be doing better".
"It's meant to have a positive effect on making sure practices have best systems in place and diagnostic ability.
"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."
Sean Duffy, national clinical director for cancer at NHS England, said: "We know that early diagnosis is the single most important factor in cancer survival, and that's why patients who visit their GP with 'red flag' symptoms like very persistent coughs, blood in urine or faeces or breast lumps should always be referred for further tests on two-week pathways.
"But not all patients visit their GP about these symptoms, and others may have cancer without developing specific symptoms.
"These patients will therefore have their cancer diagnosed after going to a hospital as an emergency, or have it spotted incidentally while receiving treatment for another, unrelated issue."
He said the data "isn't a clear measure of how good a GP practice is at spotting cancer".
"In fact, it tells us more about how the system is working as a whole."
Today's data also shows some variation across England on other diseases, such as stroke or heart failure.
While most GP practices refer patients who have suffered a stroke or mini-stroke for further investigation, some do not refer all their patients, and not all patients at risk of stroke get the right drugs.
According to NHS England, a huge number of factors can affect an individual GP practice's performance - from the make-up of its population through to the quality of its clinical staff.