Unison says charities are ripe for unionisation after securing a sole recognition deal with Barnardo's.
Employment analysts Income Data Services hailed the agreement with the children's charity, which has 6,000 staff, as "perhaps the most significant" in the sector.
A spokeswoman for Unison, which represents 42,000 voluntary sector employees, predicted that more charities will follow.
"It's a growing sector that it isn't very unionised so the potential is large," she said. "It's a sector we hope to make inroads into."
She added that pay and conditions were "pretty poor" in many charities: "Unions have a positive effect on this and morale."
Barnardo's chief executive Roger Singleton signed the deal, which followed a ballot in 2003 when staff voted in favour of representation.
Unison has a joint recognition agreement with NCH. It is also in talks with Mencap and Scope.