Plan UK works for children across a range of sectors including education, health and child protection across Africa, Asia and Latin America. Interact works across Africa and Asia supporting vulnerable peoples' right to sexual and reproductive health.
The charities were partly merged in October 2009 when Interact, which has 12 staff and an annual income of £3m, became a subsidiary of Plan UK, which has 170 staff and an annual income of £54m.
At that stage Interact retained a separate legal identity, a separate board and a separate charity number, but shared a chief executive.
The latest decision will result in Interact being fully integrated into Plan UK’s structure. It will cease to be an independent charity and its board of trustees will be removed.
A statement on Plan UK’s website said the initial merger was based on a shared focus between Plan UK's and Interact’s work on adolescent sexual and reproductive health. The move to a full merger will expand on this focus, with Interact potentially supporting work across all Plan UK country offices, increasing Interact's reach from six countries to up to 50 countries, the statement said.
A spokesman for Plan UK said that after the merger the Interact brand would become a sub-brand of Plan UK, with limited usage. He said the Interact website would be kept for six months, with a link to the Plan UK site, as part of the merger arrangements.
The spokesman said that there would be one redundancy as a result of the merger: Alan Smith, Interact’s chief operating officer.
The statement on Plan UK’s website said: "All other staff will transfer across to Plan UK if they so wish, and indeed for the most part remain working on the same projects as they do now."
Tanya Barron, chief executive of Plan UK, said in a statement: "We anticipate this move will have a positive impact on our work and beneficiaries in developing countries, allowing many more young people to benefit from quality sexual and reproductive health and rights work."