Government targets for breast cancer are inadvertently harming patients, research has warned.
Since 1999, the government has said all urgent breast cancer patients should be seen by a consultant within two weeks of seeing their GP.
Following this, research at Frenchay Hospital in Bristol suggested a "two-tier" system had developed, with up to 8,800 diagnosed as "routine" patients meaning they did not have to be seen within the two-week period.
The Bristol team found that the number of women seen urgently rose by 42 per cent overall, but the identification of "urgent" cases appeared problematic.
After tracking nearly 25,000 urgent and non-urgent breast cancer patients between 1999 and 2005, they found instances of breast cancer in the non-urgent group rose from 2.5 per cent to 5.3 per cent.
At the same time, genuine breast cancer cases in the urgent group fell from 12.8 per cent to 7.7 per cent.
By 2005, more than a quarter of women eventually diagnosed with cancer came through a non-urgent route, prompting the researchers to recommend an "urgent review" of the two-week target.
Simon Cawthorn, lead researcher for the study published in the British Medical Journal (BMJ), said government targets were effective in increasing the number of women seen urgently but doctors were struggling to decide who counted as an urgent case.
A Department of Health (DoH) spokesperson said they accepted not all patients came through the two-week rate route, which was introduced to reduce the anxiety of waiting for a diagnosis.
"The majority of those who come through routes other than their GP are detected through screening and once they are diagnosed they are treated within 31 days," they said.
The spokesperson argued NHS cancer services have been "transformed" in recent years with the vast majority of patients treated within 31 days.
The Liberal Democrats, however, said the report showed the failings of the two-week rule.
Norman Lamb, Lib Dem health spokesman, said the report was "shocking proof" of how a system with good intentions could fail patients.
He said: "It is clearly critical that cancer patients are seen very quickly but the current system actually leaves many people excluded.
"Patients waiting for such a crucial diagnosis should not be forced to rely on a crude system that is clearly letting down large numbers of people."