The new law which comes into force on Friday, means that both adults in a stable relationship can become a child's legal parents.
Until now, only married or single people could become adoptive parents so in unmarried couples only one of the adults was legally recognised.
Felicity Collier, chief executive of the British Association for Adoption and Fostering, said: "These changes will have a major impact on thousands of families.
"Opening up adoption to unmarried partners will encourage more people to consider adoption and this is very important at a time when too many children wait too long in temporary care waiting for an adoptive family."
The Adoption and Children Act 2002 also gives birth mothers and other birth relatives the right to trace children they put up for adoption once they have become adults.
They will be given the legal right to ask for an intermediary service to find the child and ask if they would like to be contacted.
Collier said: "We believe there are many adopted adults who have not made the first approach in case it is not welcomed by their birth parent but who would want to respond if they knew their birth parent was concerned to know how they were."
Other changes allow foster carers and relatives apply for special guardianship orders which are expected to last until the child is 18, giving them responsibility for the child instead of a local authority.