The number of women under 50 who are diagnosed with breast cancer has exceeded 10,000 in a year for the first time in the UK, figures show.
Cancer Research UK said it means one in five breast cancer cases are in younger women.
Figures earlier this week showed a large increase in breast cancer in Scotland, with cases in women of all ages up by almost 14 per cent in a decade.
In total, the number of women struck by breast cancer each year is now approaching 50,000 across the UK. In Scotland, around 4,500 are diagnosed with the disease each year.
But Cancer Research UK said other research showed fewer women under 50 than ever before were dying from breast cancer thanks to better treatments.
The charity said that 7,712 women under 50 were diagnosed with breast cancer in the UK in 1995. But by 2010, 10,068 were diagnosed with the disease – a 29 per cent increase in cases.
Cancer Research UK said the rise in younger women reflected the overall growth in women of all ages developing the disease.
The charity said it was not clear why rates of breast cancer were rising in this age group, but increasing alcohol intake and women having fewer children and having them later in life – factors which affect hormone levels – may be to blame. Increased use of the Pill may be playing a role, too.
Sara Hiom, Cancer Research UK’s director of health information, said: “Breast cancer is more common in older women but these figures show that younger women are also at risk of developing 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 straightaway, even if they have attended breast cancer screening.”
Despite the increased number of women under 50 diagnosed with breast cancer, the rate of women in this age group dying has fallen by 40 per cent since the early 1990s, the charity said.
In the early 1990s, the death rate from breast cancer in women under the age of 50 was nine per 100,000 women in the UK. But by 2008-10, this had fallen to five in every 100,000.
More than eight out of 10 women diagnosed with breast cancer before the age of 50 now survive for at least five years.
Ms Hiom added: “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.”