Breast cancer screening saves the lives of two women for every one who receives potentially unnecessary treatment, research out today suggests.

Some cancers grow so slowly that a woman may die from another disease first while other cancers would cause no harm.

Experts are currently unable to distinguish between these less harmful cancers and some more aggressive types, meaning they are all routinely treated.

Now new research has found that for every case of overdiagnosis, two lives are saved as a result of the NHS breast cancer screening programme.

The issue has been the subject of debate recently after other studies suggested the programme does little to save lives.

Last week, Danish experts cast doubt on the benefits of mammography, saying there were few differences in death rates between women who are screened and those who are not screened.

The latest research was led by experts from the Wolfson Institute for Preventive Medicine at Barts and the London School of Medicine and Dentistry.

It focused on data from some 80,000 women from the age of 50 and looked at Sweden and England before and after the introduction of screening.

It found 5.7 deaths from breast cancer were prevented for every 1,000 women screened over 20 years in England.

The number of estimated cases of overdiagnosis was 2.3 per 1,000 women over the same period.

The authors, writing in the Journal of Medical Screening, said: "The benefit of mammographic screening in terms of lives saved is greater in absolute terms than the harm in terms of overdiagnosis.

"Between 2 and 2.5 lives are saved for every overdiagnosed case."

The researchers concluded the "benefits in terms of numbers of deaths prevented are around double the harms in terms of overdiagnosis.

"Analysis of both data-sets shows a substantial and significant reduction in breast cancer deaths in association with mammographic screening."

Lead author Stephen Duffy said: "This shows that the benefits of screening outweigh the harms.

"Unfortunately, we haven't yet got a flawless screening test, and some cases that are picked up wouldn't have needed treatment.

"But for every case like this, screening saves two women who would have otherwise died from breast cancer."

More than 45,000 women are diagnosed with breast cancer each year in the UK and more than 12,000 die from the disease.

Women aged 50 to 70 are invited for NHS breast screening every three years across the UK. From 2012, this will be extended to include women aged 47 to 73.

Dr Lesley Walker, director of cancer information at Cancer Research UK, which part-funded the study, said: "The National Breast Screening Programme saves lives, so we encourage women to go when invited.

"All women should have access to high quality information and advice to help them make decisions that are appropriate to their individual condition.

"It's standard practice to have these discussions which help women make the choice that's right for them, so overdiagnosis need not be a reason to feel worried about going for screening.

"As well as attending screening, we would encourage all women to be breast-aware and to go to their doctor if they notice any unusual changes to their breasts."

Dr Alexis Willett, head of policy at Breakthrough Breast Cancer, said: "This research provides a compelling argument in favour of breast screening and shows that the benefits outweigh the risks.

"Screening saves lives because it picks up cancer before any symptoms can be seen or felt.

"The earlier cancer is diagnosed the better a woman's chance of a successful outcome."

Arlene Wilkie, director of research and policy at Breast Cancer Campaign, said: "We welcome continued evaluation of the effectiveness of the NHS Breast Screening Programme as we believe it plays a valuable role in the early detection of breast cancer.

"Research has shown that the earlier breast cancer is detected and treated the greater the chances of survival."