A Breakthrough spokeswoman said there would be no job cuts and extolled the benefits of the move, with UKBCC bringing grassroots advocacy experience to the higher profile and funding enjoyed by Breakthrough.
UKBCC, which was set up in 1995 and has a network of around 500 local advocates, is renowned for its annual lobbying and training event Westminster Fly-In.
Breakthrough is currently using supermodel Jodie Kidd as the face of its Fashion Targets Breast Cancer campaign.
Breakthrough Breast Cancer chief executive Delyth Morgan said: ‘Merger gives Breakthrough an additional wealth of grassroots patient representatives with personal experience of the disease, who can inform the development of our national campaigns.”
Around 40,000 women are diagnosed with breast cancer every year.
The charity enjoys widespread corporate support from companies such as cosmetics giant Avon, which has donated £8m so far, Ford and Marks & Spencer.
Earlier this year Breakthrough unveiled a 29 per cent rise in annual income to £7.3m for the year to July 2002.
UKBCC members will be asked to vote on the merger at an Extraordinary General Meeting on 8 November
Rival charity Breast Cancer Care, which focuses purely on providing information and support, raised £5.2m last year.