Both charities focus on research, with Breakthrough Breast Cancer running its own research centre, and Breast Cancer Campaign funding scientists through grants.
The new, merged charity has yet to be named, but will face a challenge differentiating itself from rivals such as the Pink Ribbon Foundation and Breast Cancer Care.
There’s also competition from bigger players such as Cancer Research UK and Macmillan, though neither specialise in breast cancer.
Fiona Hazell, director of communications for Breast Cancer Campaign, told Marketing one of the most important goals was to eradicate "confusion" about how the public can support breast cancer research.
She said: "The biggest challenge facing us is creating a single-minded proposition that affects all those affected by breast cancer, which works across multi-channels and sits at the heart of our fundraising.
"We are targeting everybody in the UK who is, or has been, affected by breast cancer, as well as researchers looking for funding and supporters helping us to spread the word and raise funds."
Hazell name-checked Macmillan as a particularly strong player in an already crowded market. A key difference, however, is that Macmillan provides support to existing cancer sufferers, rather than focusing on prevention.
She said: "All charities face differentiation challenges, of course, and the cancer charity market in particular has a number of very high profile, very strong players."
Currently, a team comprising employees of both charities has been tasked with building the new brand with the help of agency The Clearing. It isn’t clear how the marketing team at the new brand will look.
Hazell said: "The intention behind the merger is not to reduce headcount but we are, of course, duty bound to ensure we’re making the most of supporters’ money.
"Where we find that there’s duplication, unfortunately we will have to make redundancies to ensure that we are using supporters’ money as efficiently as possible."
She said the team had drawn inspiration from existing brands, such as Prostate Cancer UK, whose nimble, "single-minded" marketing efforts have successfully boosted public interest in men’s health.
Jonathan Hubbard, creative director at The Clearing, said the new charity would likely rely heavily on emotive messaging.
He said the charity would prioritise disabusing the public of the notion that breast cancer research was "a done deal".
He said: "Breast cancer is still out there, and it’s devastating, and that’s what this charity needs to stand for."
Budget limitations means the charity will prioritise digital channels and its existing communities, rather than traditional media, Hubbard added.