Health experts are warning that an "alarming" number of women suspected of having breast cancer are waiting too long to be diagnosed.
In 1999 the Department of Health introduced a "two week wait rule" for suspected breast cancer cases that are deemed urgent, meaning women in this category should see a consultant within 14 days of seeing their GP.
But experts are calling for the system to be reviewed, after a study found a rising number of breast cancer cases among women who are classed as "non-urgent".
The research, carried out at the Frenchay Breast Care Centre in Bristol, has been published ahead of print in the British Medical Journal, and involved 24,999 referrals for patients made between 1999 and 2005.
While the number of patients referred under the two week rule increased over the period, the percentage eventually diagnosed with cancer "significantly decreased" from 12.8% to 7.7%.
But despite a fall in the number of non-urgent referrals during the same period, the percentage of cancer cases in that group more than doubled to 5.3%.
The authors of the report concluded: "If breast cancer services are to be improved, the two week wait rule should be reviewed urgently."
Cancerbackup senior nurse Kim Hardwick said: "The delay in treatment for those women referred as non-urgent that prove to have cancer could have an impact on treatment outcomes.
"Cancerbackup is calling for all people with suspected cancer of any type to be seen within two weeks."