Fair Play for Children, a charity that campaigns for children's right to play, claims that a proposal to admit a trade union would be another step towards placing the council in direct competition with its own members.
At a meeting today (9 June) the council, which brings together voluntary groups involved in play, will consider a request for membership by the Community and Youth Workers Union.
Fair Play's national organiser Jan Cosgrove argued this went against the council's founding principle of non-competition with its constituent bodies.
In a paper to the council he said: "It seems unfair that the council, with government grant and special access, facilities and status, should be, in effect, competing on a uneven playing field with its member organisations."
He said that "the only sound course" was for Fair Play to withdraw, describing the move as a last resort in a long-running battle to curb the expansion of membership categories.
The Children's Play Council was formed in 1986 out of a collaboration between Fair Play, the National Playing Fields Association and Noosa, now 4Children. The NPFA has since left.
Cosgrove claimed Fair Play's departure would place a question mark over the council's ability to represent the sector.
However, Tim Gill, the council's director, said: "It is very common among the voluntary sector for organisations to have overlapping membership."
He said that it would be up to members whether or not to explore the concept of allowing a union to join.
If they agree, then a proposal to change membership criteria could be put forward.
He added: "We are sorry that the dispute has arisen. Fair Play has been historically very independent and forthright, bringing to our membership a valued perspective, and we would be sorry to lose that."
