The creation of the new RAA, Coram Ambitious for Adoption, is part of the government's ongoing drive to introduce RAAs across England.
It aims to simplify the adoption process and ensure that the voluntary sector and local authorities are collaborating as effectively as possible to speed up matching of children and approved adopters.
Nadhim Zahawi, children and families minister, said that more than half of all councils in England are now delivering adoption services via RAAs, and welcomed London's first.
The agency will provide adoption services for several local authorities across Greater London, starting with Harrow, Redbridge and The City of London.
It will be followed in the coming weeks by others including Bromley, Waltham Forest and Slough Children's Services Trust.
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It hopes to boost approvals after data published by the Adoption and Special Guardianship Leadership Board last month showed a 27 per cent fall in London adopter approvals as of December 2018 compared with the previous year.
The gap between the number of children awaiting adoption and approved adopters also doubled over the same period - there were 220 children waiting and 160 approved adopters.
In addition to concerns about the impact on children's long-term outcomes the longer they wait to be placed, analysis by Coram suggests there is also a high associated cost.
Coram claims that failing to find adoptive families for just 30 children a year costs £1m for each year they remain in care.
All local authorities across the country are expected to deliver their adoption services through an RAA by 2020.
Coram's chief executive Carol Homden said: "Given the current challenging climate of adoption, there has never been a more important time for those in the sector to work together to combine skills and resources and match children and adopters in a timely way.
"We urgently need adopters to come forward, and to achieve that means providing timely and effective support for adopters and for children.
"We would urge anyone who feels they can offer a safe, secure and loving family for a vulnerable child to contact us to find out more."
Paul Hewitt, corporate director of people services for Harrow Council, which is leading the regional agency's development, said: "Together we share a commitment to improving outcomes for children and are ambitious for adoption for those children who need it."
Coram Ambitious for Adoption is hosting free monthly information events in London for those interested in adoption to find out more about the process.
In addition to the Big Adoption Day today, which took place on 5 June, further events are taking place on 10 July and 12 August 2019.