Nadine Flood has announced she is stepping down as national secretary of the Community and Public Sector Union, citing health reasons.
After ten years leading the main public sector union, Ms Flood has told staff and members it was a difficult decision to step down but she needed to listen to her doctors.
"At the moment, doing this job means I can't take care of my health the way I need to, and I've decided it's time to take the advice of my doctors and step back," she wrote in an email sent on Friday afternoon.
"I'm hopeful that if I stop ignoring my doctors' advice I can look forward to getting well and going on to do work that helps working people and keep fighting for the values I hold dear."
Under the union's rules the governing council will meet in three weeks to appoint an acting national secretary, before a permanent national secretary is elected next year. Ms Flood will stay on to help the transition to the new national secretary before stepping back.
Serving in the role of national secretary was "an incredible privilege," Ms Flood said.
"Representing members on the big issues affecting your jobs and work, and on the everyday issues in your workplace, has been an absolute privilege," she wrote.
She said the union had strong succession planning in place.
Staff were upset by the announcement on Friday, and assistant national secretary Michael Tull paid tribute to Ms Flood as a "phenomenal leader".
"Over the last nine years and six prime ministers, she has led the union as we fought for better wages, conditions, jobs, and for our community's services. Her defence of our members and the public service has always been fearless and forthright," he said.
"She will leave behind her a united Executive and a union that is well placed to continue the work of representing our members in their workplaces and in the community.
"While we are sad to see her go, it's important she place her health first and we respect her decision, which we know has not been an easy one."