Guide Dogs, which employs about 1,200 staff, provides support to blind and partially sighted people and had an income of £66.1m in 2011. NBCS, which works with visually impaired children, has an annual income of about £1m and employs 30 staff.
In September, the two charities signed a memorandum of understanding that involved them sharing skills and expertise. Since signing the memorandum, the charities say, they have grown closer together, and NBCS became a subsidiary of Guide Dogs on 29 April.
In a statement, NBCS said it no longer had "the size or the resilience at a time of recession to continue to develop its services", leading to the decision to join forces with Guide Dogs.
Although a subsidiary of Guide Dogs, NBCS remains as a separate legal entity. It will retain its own branding and will continue to fundraise and deliver services as NBCS.
Asked whether the move would result in any job losses, a spokeswoman for Guide Dogs said that a consultation among NBCS staff on a new structure would begin in the first week of May and would last for 30 days.
A final decision on the future structure of the charity would be made after the consultation, she said.
"Guide Dogs’ aim is to continue to develop and improve services for children and young people who are blind or partially sighted and their families, so the expertise, experience and knowledge of NBCS’s staff will be vital in achieving our aspiration," said the spokeswoman.
The charities will continue to have separate boards and three NBCS trustees will also become trustees of Guide Dogs.
Richard Leaman, chief executive of Guide Dogs, said: "This is an incredibly powerful development – if we get this right and we gain public support for our work, we will change forever the landscape for children with sight loss in the UK and enable them to enjoy the future they all deserve."
Carolyn Fullard, chief executive of NBCS, said the arrangement would allow her charity to continue to provide its services to children and young people. "This partnership secures the financial future of NBCS and means that we will be able to fulfil our ambitions to reach many more children and young people who are blind or partially sighted and their families, as well as expanding the range of our services."