The disease was also more likely to come back.
The findings came from analysis of 400 previous studies and aimed to discover which patients were more likely to develop advanced breast cancer.
The team found that women under 35 had a 12.7 to 38 percent chance of developing secondary – metastatic – cancer. That contrasts with 3.7 to 28.6 percent among those 50-plus. The overall risk was six to 22 percent. Former Girls Aloud singer Sarah Harding, 39, died in September after revealing last year she had breast cancer which had spread.
Early diagnosis of what is now the world’s most common cancer can save lives.
But experts say many younger women do not even realise they are at risk.
Around 1,000 in the UK die each month from incurable secondary breast cancer. Dr Eileen Morgan, of the International Agency for Research on Cancer, said the under-35s may be at greater risk of metastasis because they have a more aggressive form or are diagnosed at a later stage.
She said: “Most women are diagnosed when it is confined to the breast or has only spread to nearby tissue. But in some, the cancer will grow and spread to other parts of the body or come back in a different part several years after the end of their initial treatment.
“At this point the cancer becomes much harder to treat and the risk of dying is higher.
“However, we don’t really know how many people develop metastatic breast cancer because cancer registries have not been routinely collecting this data.”
The researchers also found women with a breast cancer called luminal B had a higher risk of secondary disease compared with those suffering the luminal A form.
But they also learned the chance of breast cancer returning after initial diagnosis and spreading to other organs appeared to decrease over time for those first diagnosed in the 1970s and 1980s.
Kotryna Temcinaite, of Breast Cancer Now, presented the findings at the Advanced Breast Cancer Sixth International Consensus Conference in Portugal’s capital Lisbon.
She said: “We don’t currently know who will develop secondary breast cancer and when, but this analysis provides helpful insight into who is most at risk.”
Awareness charity CoppaFeel! launched a Know Yourself campaign aimed at younger women after Sarah’s death. Breast cancer is much more common in older ages but around 5,000 under-45s are diagnosed every year.
Signs include a lump or thickened area, swelling or lump in the armpit, a change in nipple appearance and new puckering or dimpling.
- Breast Cancer Now’s free helpline is on 0808 800 6000.