The general secretary of the University and College Union, Sally Hunt, has resigned for health reasons.
The union’s national executive committee will meet on Friday to determine the process for the election of her successor.
More news: Sally Hunt wins UCU re-election
'One of the leading female union leaders'
UCU president Vicky Knight said: "The union owes a huge debt of gratitude to Sally. She was central to the creation of UCU and has led its growth and success with utter dedication to the interests of the members that we represent, and with an integrity that I know is widely acknowledged and admired throughout the movement and beyond."
"She has been one of the leading female union leaders of her generation and has inspired many others by her example. We will all miss her greatly and I wish her the very best for the future."
Sally Hunt: 'It has been a real honour'
Sally Hunt told UCU members and staff the union would "continue to be a great union; a reflection of its truly wonderful membership who contribute so much to the culture, politics and economy of this country".
"It is no surprise then that the union itself is vibrant, energetic and stimulating. I am grateful for the trust given to me as their general secretary and wish every UCU member and staff member well. It has been a real honour. Thank you."
TUC general secretary Frances O’Grady said: “Sally was a fantastic president of the TUC in our 150th year and continues to be a great champion of education and lifelong learning, open to all. A strong woman in a tough job, Sally has been a source of inspiration and support to me and many others right across the trade union movement."