Women under 50 are being diagnosed with breast cancer at the record rate of 27 a day, reaching 10,000 a year for the first time.
One in five cases now involve women in this age bracket, while the rate is still climbing among older women.
Experts fear modern lifestyles are to blame for the rise among younger women. Many are drinking excessively, overeating and failing to exercise – all habits which contribute to breast cancer.
The growing trend for women to delay having children until their 30s and 40s, have smaller families and breastfeed for short periods of time, if at all, also pushes up the risk, claims Cancer Research UK.
The charity suggested that increasing use of the contraceptive pill may also play a role in the rise.
Breast cancer kills around 12,000 women annually, but more than ever before are surviving due to advances in treatment.
In women under 50 the death rate has almost halved in the last 20 years.
Chris Askew, chief executive of Breakthrough Breast Cancer, said: ‘Although breast cancer is more common in older women, it’s worrying to see an increase in the number of younger women diagnosed with the disease.
‘More women than ever are surviving which is great news, however more women are getting breast cancer and we must invest in vital research for new treatments and disease prevention.’
Statistics released by Cancer Research UK show that around 7,700 women under 50 were diagnosed with breast cancer in 1995 in the UK, but the figure had risen to 10,068 in 2010.
The increase in breast cancer in women of all ages over the same period was 18 per cent.
In younger women, a family history of the disease increases the risk.
But experts are concerned about changing lifestyle patterns among younger women, with many choosing to have children later or remain childless, which boosts the risk.
Pregnancy before the age of 30 and breastfeeding cuts a woman’s lifetime number of menstrual cycles, thereby reducing overall exposure to oestrogen, a hormone which drives most breast cancer tumours.
Previous research suggests women who breastfeed for six months reduce their risk of dying of cancer by ten per cent, possibly by a direct effect on breast cells making them more resistant to cancer.
Age remains the strongest factor for breast cancer, with a healthy lifestyle cutting risk at any age.
Scientists estimate four out of every ten cases in the UK could be prevented through maintaining a healthy weight, drinking less alcohol and being more physically active.
Women who are overweight or obese run a higher risk of developing the disease, probably through changes in sex hormone levels triggered by weight gain.
Studies show drinking just one large glass of wine a day increases the chances of developing breast cancer by a fifth.
Cancer Research UK says the contraceptive pill slightly increases the risk while women are taking it, and it has become more popular.
Sara Hiom, the charity’s director of health information, said: ‘The number of cases in women under 50 diagnosed with breast cancer is increasing slowly, but thanks to research, awareness and improved care, more women than ever before are surviving the disease.
‘Women of all ages who notice anything different about their breasts, including changes in size, shape or feel, a lump or thickening, nipple discharge or rash, dimpling, puckering or redness of the skin, should see their GP straight away, even if they have attended screening.
‘It’s more likely not to be cancer but if it is, detecting it early gives the best chance of successful treatment.’
Mia Rosenblatt of the Breast Cancer Campaign said: ‘This increase in cases among younger women is a pattern evident across all ages.
‘It is vital that the particular concerns of younger women, such as fertility issues as a result of treatment, are addressed and that specialist support is provided by clinicians.
‘Apart from being a woman, age still remains the biggest risk factor for breast cancer, with 80 per cent of cases in women over the age of 50. However, no matter what their age, all women need to be breast aware.’