An "alarming" number of patients with suspected breast cancer are waiting too long for a diagnosis, doctors warn.
Breast cancer symptoms should be assessed urgently
Government targets dictate all suspected breast cancers should be seen by a specialist within two weeks.
But a team at Bristol's Frenchay Hospital discovered an increase in the number of positive diagnoses among women deemed to be non-urgent cases.
Doctors said the target was failing, but the government said it was looking to improve the situation.
The study published in the British Medical Journal reported that the two-week wait target - from GP referral to consultant appointment - was introduced in 1999 because of long waiting lists for diagnosis and treatment.
Doctors at the Breast Care Centre at Frenchay Hospital compared nearly 25,000 urgent and non-urgent referrals - where breast cancer is not suspected but a consultant's opinion is still needed - between 2000 and 2005.
They discovered that the number of women referred urgently by their GP had increased, as expected.
But the proportion of cancers detected in those seen within two weeks went down from 12.8% to 7.7%.
Meanwhile, the numbers seen as non-urgent cases fell, but the proportion diagnosed with cancer rose from 2.5% to 5.3%.
Lead researcher Simon Cawthorn said the target had been very effective in getting many women seen quickly but that most of the time it was impossible to tell whether a breast lump was cancer or not.
"The message is that we need to see everyone within two weeks.
"Even though it's only a small number in the routine group, it's a significant number."
He added that because the two-week wait rule had improved diagnostic services, GPs were now referring women they would have previously asked to come back to see them in a month or two.
"They are having to decide whether it's urgent or not and the thing is you just can't tell."
The team have now invested in two specialist breast nurses and see all patients within two weeks.
A Department of Health spokesperson said ministers were looking to improve the situation.
"We accept that there will always be some patients who do not come through the two-week wait route, because they do not have obvious symptoms, are detected through screening, or through investigation for other conditions.
"In 2005, the government made a manifesto commitment to go further on cancer waits and we are considering proposals to do this as part of the Cancer Reform Strategy due to be published at the end of this year."
Maggie Alexander, director of policy and campaigns at Breakthrough Breast Cancer said all patients should be seen within two weeks
"We have known for some time that many women eventually diagnosed with breast cancer are given a routine referral by their GP, and as a result may endure anxious, long waits to find out if they have breast cancer."
Hisham Hamed, Cancer Research UK breast surgeon, said the aim was for all patients to be seen in the shortest possible time and the majority referred did not have breast cancer.
"It is important to say that research shows an extra week or two will not compromise the patient's outcome."