Dawsongroup chief executive Mike Williams has stepped down due to underlying health reasons.
Williams had been with the company for 40 years, becoming managing director of Dawsonrentals in 1979 and rising to the chief executive role with the group in 1993.
Peter Dawson, group chairman, said, "It is a simple commercial truth that nothing stays the same: change is inevitable.
"After working so closely with Mike for 40 years however, it is hard to accept.
"His energy, commitment and commercial ability have been instrumental to the development of the business we have today.
"As has his very individual, slightly acerbic sense of humour; used so often to very good effect as he made his points both to suppliers and his own team alike.
"However, he would be the first to acknowledge the huge part played by the input and support of that team, a number of whom have been with him for many of those 40 years.
"I am sure I speak for all who know Mike when I wish him a full and speedy recovery."
Williams added: "I had been planning my eventual retirement from the business for some time and had hoped to finish toward the end of 2017.
"However, this new situation has been forced on me by the need to deal with a health matter that fortunately, I am being told, should be managed successfully.
"Sadly, the time this may take means my retirement plan is not going to work, so it is with real regret and a sense of unfinished business that I must now complete my time with Peter and the team.
"It's been good, very good at times, but as the saying goes, all good things come to an end."
Steve Miller, who has worked alongside both Mike Williams and Peter Dawson for 30 years, the last seven as group managing director, has been announced as Williams' successor.