One in three cases of breast cancer are in women aged 70 and over - an age group that is not routinely screened, new figures show.
Around 13,500 women aged 70 and over are diagnosed with breast cancer in England every year, according to data from Public Health England (PHE), which is launching a campaign to highlight the issue.
Those aged 70 and over account for more than half of all breast cancer deaths every year, with around 5,400 women in this age group dying from the disease.
Survival rates among the over-70s are generally lower than in younger women and, compared with countries such as Sweden, the UK has far more older women dying from breast cancer.
At present, t he breast screening programme in England invites women for screening every three years from age 50 to 70.
This routine screening programme is being extended up to the age of 73, with roll-out planned across England by 2016. Women over 70 currently have to ask if they want screening.
In Scotland, women are screened routinely up to the age of 71. Women aged 71 and over can only be screened if they self-refer to their local screening centre.
Meanwhile, in Northern Ireland, only women aged 50 to 70 are invited for screening. The same is true for Wales.
PHE is launching a new Be Clear on Cancer campaign to tell older women "don't assume you're past it".
Women are being urged to visit their doctor if they spot any changes in their breasts. Experts believe a lack of awareness of symptoms that are not a lump - such as changes in the shape or size of the breast - could be one of the reasons older women do not seek help.
A poll of 1,500 women of all ages found that 67% of those aged 70 and over incorrectly think women are equally likely to get breast cancer regardless of their age. In fact, a woman's risk of breast cancer increases with age.
England's c hief medical officer, Professor Dame Sally Davies, welcomed the campaign.
She said: "Survival rates from this disease decrease with age; however, awareness of symptoms and risk is low amongst this age group, meaning these women are more likely to be diagnosed at a late stage. The earlier breast cancer is diagnosed, the better the chance of survival."
Sean Duffy, national clinical director at NHS England, said: "When we look at other countries such as Sweden it is clear that we are losing far too many older women to breast cancer. In 2009, it was estimated that around 2,000 deaths from the disease could be avoided each year in England if survival rates matched the best in Europe.
"Whilst we have made good progress in the last decade, we are still lagging behind our international counterparts. This latest Be Clear on Cancer campaign has an important role to play in helping increase symptom awareness levels, early diagnoses and, ultimately, survival rates."
More than 90% of all women diagnosed with early-stage breast cancer survive for at least five years, but this drops to 15% for those whose cancer is in the later stages.
Caroline Abrahams, charity director of Age UK, said: "Older women are less aware of cancer symptoms, and research shows that doctor are less likely to offer older patients the most effective treatments, despite the fact they can benefit.
"Older women need to be aware that developing breast cancer in later life is a real possibility and that with the right support, treatments can be successful whatever your age.
"Furthermore, it's critical that decisions about the care of older people must be based on their needs and not simply on their age."
Actress Barbara Windsor, 76, said: "I met a lot of women affected by breast cancer when I was preparing for Peggy Mitchell's diagnosis in EastEnders, which made me realise just how important an early diagnosis is.
"You get to a certain age and think you're too old for some things, but breast cancer isn't one of them."
TV and radio presenter Gloria Hunniford, 73, said: " I know first hand, having lost my daughter Caron, the impact breast cancer can have on people's lives, and the importance of checking for symptoms.
"The earlier breast cancer is caught, the higher the chances of survival. So know the symptoms, check regularly and visit your doctor if you are concerned. Don't just look out for yourself, you can also play a key role in encouraging those close to you to do the same."
Sara Hiom, director of early diagnosis and patient engagement at Cancer Research UK, said: "The sooner that breast cancer is diagnosed, the more likely treatment is to be effective and the chances of beating it are greater - whatever your age."
Professor Julietta Patnick, director of the NHS Cancer Screening Programmes, said: "All women over 70 are still entitled to be screened every three years on request and we would encourage those who wish to be screened to contact their local breast screening unit.
"Women are given cards, which are handed out at their last routine breast screening appointment, to help them remember."