A statement from the charities said Prospects had become a subsidiary of the Livability group from yesterday.
It said the move would enable the organisations to "build capacity, enhance delivery for the people we support, grow disability awareness, mobilise a broader supporter base and maximise our effectiveness and sustainability".
Livability provides services including residential care, a range of supported and independent living services, a brain injury rehabilitation centre, a school and a further education college for disabled people.
Prospects provides support for people with learning disabilities and their families and runs care homes. It employs about 400 staff and had an income of £11.7m in the year to the end of March 2015.
Livability, which was formed in 2007 by the merger of John Grooms and the Shaftesbury Society, employs about 1,600 people and had an income of £44.4m over the same period.
The Prospects brand is expected to be dissolved as it is integrated into the Livability group over the next two years, a Livability spokeswoman confirmed.
She said it was too early to say whether there would be any job losses or redundancies as a result of the merger, but said there would be no immediate changes and both charities’ services would be continuing as usual.
Dave Webber, chief executive of Livability, said: "Prospects joining Livability is a wonderful opportunity for impact and change.
"It’s certainly not news that disabled people in the UK are too often at the brunt of cuts to public services and shrinking budgets. They often face increasing social exclusion, health limitations and poorer life opportunities. Charities like ours must mobilise local people, resources and strengths to take the lead and work together for lasting, positive change across our neighbourhoods."
Mick Ledden, chief executive of Prospects, said: "Delivering the best possible service and experience for the people we support is top priority for our charities. With such an alignment of ethos and strategic focus, joining our operations and expertise with Livability will enable us to enhance, sustain and deepen our work."