RAPt, which has an annual income of £14.3m and employs 338 people, according to figures on the Charity Commission website, is assuming company membership and thereby control of Blue Sky, which has an income of £1.7m and 17 employees.
RAPt is becoming Blue Sky’s sole company member and replaces Groundwork South, which has been Blue Sky’s parent body since it was formed in 2005.
Blue Sky will remain a separate legal entity and retain its own head office as part of the move.
Mick May, chief executive of Blue Sky, is standing down at the end of the year on health grounds, to be replaced by a new managing director.
One of Blue Sky’s senior management team will also step down but there are no planned redundancies. Mike Trace will remain chief executive of RAPt.
Blue Sky’s governance structure will be refreshed after the merger. RAPt will make four nominations to the Blue Sky board, with Blue Sky nominating the remaining four trustees.
May said that the joining of the two organisations was "driven by strategic thinking, not financial need".
He said: "It will produce a unique offering: no other organisation offers this continuum of support, from in-prison care for addicts to resettlement support through the prison gate and into a proper paid job with a proper company on the outside."
Trace said that by working together the two organisations would create something "wholly unique" that could generate even stronger results.
"Three factors informed this groundbreaking merger," he said. "First, drug and alcohol abuse is the greatest driver of crime in the UK. Second, having a stable job reduces the probability of re-offending by up to 50 per cent. Finally, the best rehabilitation takes place both sides of the prison gate."