Keller said that Alain Michaelis had decided to step down as chief executive and as a director.
He is not being required to serve much notice period – the board has told him he can go at the end of next week.
Chief financial officer Mike Speakman will become interim CEO from 1st October 2019 and financial controller Mark Hooper will become interim CFO.
Alain Michaelis was appointed chief executive of Keller in May 2015. He was previously group operations director of Rolls Royce.
Chairman Peter Hill said "Alain has done a tremendous job over the last four years. The Board and I would like to thank him for his enormous contribution in evolving Keller into the more integrated and professional organisation it is today and overseeing what will be the transformation of Keller North America. Having provided these foundations for Keller's future success, Alain has decided now is the right time to step down and allow his successor to lead the next stage of Keller's development. The board looks forward to working with the executive team as we focus even harder on driving financial performance and growing shareholder value."
Mr Michaelis said: "It has been a privilege to lead this great and unique company through an important phase of our evolution. I wish all those in the whole Keller community the best for the future."