Last month, the RSPCA was asked by the Charity Commission to review its governance arrangements. Three trustees have stepped down from the charity's board in recent months amid concerns about its management and governance.
In a statement on its website, Unite, which said it represents more than half of the RSPCA’s 1,600-strong workforce, said a radical governance overhaul at the charity was required.
The union’s statement highlighted concerns about the size of the RSPCA’s 25-strong board, which the union said was too "large and unwieldy", and the length of time some trustees have served, some having been on the board for decades.
The union said the board needed more transparency and independent evaluations of its performance.
The charity has been without a permanent chief executive since the resignation on health grounds of Gavin Grant in February 2014, which Unite said had "caused the organisation to suffer from strategic drift".
David Canavan, the vice chair of the charity, has been the acting chief executive while a permanent replacement for Grant is sought.
Two trustees, Christopher Laurence and Sally Phillips, stood down in recent months, citing concerns about the RSPCA’s management and governance.
A third trustee to resign, Karen Harley, who was the RSPCA’s treasurer, said she feared her reputation would be damaged by internal wrangling at the charity.
Jamie Major, a regional officer at Unite, said: "After two years of protracted pressure, there is finally going to be an external review of the governance of the RSPCA.
"This is a crucial opportunity to bring the organisation kicking and screaming into the 21st century by modernising its outdated, ineffective and stale governance structures.
"However, the lack of a chief executive for the past two years has resulted in a loss of leadership and deteriorating morale which has been exacerbated by the poor quality of trustee decision-making that exists on the RSPCA governmental board.
"Unite is calling on the RSPCA to use this review as an opportunity to overhaul its board and committees to ensure it has the right people from a wide range of backgrounds with the necessary skills to lead the organisation forward.
"RSPCA staff work really hard to look after the welfare and safety of animals. We want a board in which we can be confident will also look after its staff."
A spokeswoman for the RSPCA said: "The RSPCA is committed to ensuring that its governance arrangements remain appropriate for a leading national charity and that the council as the charity’s governing body continues to provide effective leadership and accountability for the RSPCA so we can fulfil our vital mission to prevent cruelty to animals and improve their welfare."