Bob Vickers had previously tendered notice of resignation after two years in the job and left the group by mutual consent on 30th June 2019.
Having led a restructuring of the group, overseen changes to the senior management team and created a new operating platform for each of the company's business units, Mr Vickers decided that the next phase of the group's development required different leadership, the company said.
Chairman John Clugston expressed disappointment at his departure but said he was grateful for the contribution he had brought to the privately-owned company.
Bob Vickers joined Clugston in February 2017 when Stephen Martin left to become director general of the Institute of Directors. Mr Vickers joined from Carillion, where he was a director of Carillion Construction Services.
Interim chief executive Glynn Thomas (pictured below) was corporate development director of defence contractor VT Group until its 2010 takeover by Babcock. Since then he has worked as a freelance business consultant, thorugh his company J&CE, with clients including The Hospital Company (Liverpool) Ltd, the former PFI developer of the Royal Liverpool Hospital, who brought him on board last year after the collapse of Carillion left the project stranded.