Voice, which was founded in 1975 in response to the high-profile death of seven-year-old Maria Colwell, who was murdered by her stepfather, will from today join the Coram group under the name Coram Voice.
Voice, which provides an advocacy service and campaigns on behalf of children and young people in care, will remain a separate legal entity and retain its current board of trustees.
The charity, which had an annual income of £2.4m in 2011/12, has 41 employees, 103 freelance staff and 30 volunteers working in England.
Coram, which finds adoptive families for children and provides supported housing for people who are leaving care and young people facing homelessness, has an annual turnover of about £11m and employs 213 full-time staff.
Coram Voice will relocate within London from its base in Islington to the Coram site in Bloomsbury.
Andrew Radford, chief executive of Voice, said that joining the Coram group would enable Voice to become more efficient and allow it to help more vulnerable children and young people.
He said no redundancies were planned as a result of the move.
"There are positive and negative driving forces to the merger," he said. "Like other children’s charities, we are being hit by the cuts – there is less money and the local authorities we provide services for are expecting us to do more for less.
"But there is a real positive aspect to the amalgamation. Because of the synergy between Voice and other parts of the Coram group, we will be able to combine our different experiences and skill sets to tackle the problems that children are increasingly facing. The more we can fight their corner the better."
Carol Homden, chief executive of Coram, said: "We warmly welcome our colleagues at Voice to the Coram group. This is crucially important at a time when vulnerable children and young people are feeling the effects of economic pressure and the implications of changes to legal aid, and we look forward to making an even greater difference by combining expertise to improve more lives."