A joint statement from the charities released today said they planned to form a new organisation next spring, which they said would become the UK’s largest breast cancer charity.
The two organisations said they had similar strategic aims and the move would enable more funds to be invested in research into breast cancer prevention and treatment.
The charities will spend the coming months working on a new name and look for the merged organisation, a process that will involve both staff and key supporters, the organisations said.
Fiona Hazell, director of communications at Breast Cancer Campaign, said: "While we will look different, we will remain a dedicated breast cancer research charity committed to funding breast cancer research and also to influencing for necessary change in how we tackle and treat breast cancer, as we both currently do."
Breast Cancer Campaign has 86 staff and expects to record an income of £12m in the year to June 2014, said Hazell. Breakthrough Breast Cancer employs 126 people and had an income of £16.3m in the year to July 2013.
Breakthrough spent almost £4.4m more than its income in 2011/12 and 2012/13 combined, its accounts on the Charity Commission’s website show.
Asked about the deficit, a Breakthrough spokesman said: "In common with many charities, Breakthrough’s income fell due to the 2008 global financial crisis. In response, Breakthrough’s trustees approved operating deficits over recent years to maintain the level of our charitable expenditure, and investment in short and medium-term fundraising with a view to growth in our charitable expenditure in the future."
The organisations will begin the process of appointing a chief executive for the merged charity in the new year.
They said that Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, and Chris Askew, chief executive of Breakthrough Breast Cancer, would work jointly on the merger until the appointment was made. It is not yet clear whether either or both of them will apply for the new role.
The merger will result in some redundancies, but the exact number will not be known until the new organisation’s structure is finalised.
Six of the nine directors across the two charities have been appointed to roles at the new charity, while one post is yet to be appointed to. The other three directors will become redundant once the merger takes place.
"The aim is to create a breast cancer research charity of scale by bringing people, resources and ideas from both charities together in order to end breast cancer," said Hazell. "The merger is not being done to reduce headcount, though we expect there to be some redundancies as a result."
The charities will begin the process of appointing a chair of the merged charity in January.
Isla Smith and Stephanie Monk, chairs of Breast Cancer Campaign and Breakthrough Breast Cancer respectively, will stand down once the merger takes place.
In a joint statement, Askew and Morgan said: "United, we will be a stronger, more effective force committed to putting an end to breast cancer.
"Everyone affected by breast cancer deserves to see an end to this devastating disease. By combining our energies and expertise, we’ll make greater progress in more efficient ways and achieve our shared ambition that, by 2050, no one will die from the disease."
Staff from the two organisations are expected to move into new offices in March.