Screening women for breast cancer could result in a 10 per cent rate of over-diagnosis, a Swedish study has found.

Over-diagnosis is defined as cases of cancer that would never have come to clinical attention without screening.

Although it is widely agreed that breast screening can reduce deaths, more discussion around this negative side effect of screening is needed, say the researchers.

The researchers from Malmo University Hospital used data from a large breast cancer screening trial in Sweden, carried out between 1976 and 1986.

The women were then followed until December 2001, to track survival and detection of breast cancer.

The research team found that the rate of over-diagnosis of breast cancer was 10 per cent in women who were screened at age 55-69 years, compared with an unscreened control group.

Earlier studies on over-diagnosis have shown rates of up to 54 per cent but a more recent study had put it at about 1 per cent.

However, none of these earlier studies was based on direct observations, as this new study was, the researchers said.

Lead researcher Dr Sophia Zackrisson said: "It is widely agreed that screening using mammography can reduce mortality in breast cancer.

"The rate of over-diagnosis is another issue to be considered in the ongoing discussion about clinical and public health implications of breast cancer screening."

The study findings are published in the British Medical Journal.

Breast cancer facts:

Research conducted by Imaginis shows that breast cancer is the second leading cause of cancer deaths in women (after lung cancer) worldwide and is the most common type of cancer found among women in the world.

According to figures from the World Health Organization, around 1.2 million people worldwide were diagnosed with breast cancer in 2005.

Earlier detection and better treatments have improved mortality rates in recent years.

The majority of breast cancers will be treated with surgery to remove the tumour.