The shock parting of ways was confirmed after rumours of the split spread at Dundonald Links in the build-up to the Aberdeen Asset Management Scottish Open.
The reason behind the split is understood to be a legal dispute between Westwood and the Cheshire-based company.
The development is as much of a surprise as Phil Mickelson and his long-time caddie, Jim “Bones” MacKay, recently parting company.
Along with Darren Clarke, Westwood was one of Chandler’s first signings after the former European Tour player founded his agency, with all three becoming close friends.
Westwood recorded 42 worldwide wins as part of the ISM stable, which is now left with Clarke, Danny Willett, Louis Oosthuizen and Paul Lawrie as its leading lights.
Westwood, who is tipped for a future Ryder Cup captaincy, now lives in Edinburgh.