Riddle had been at the agency since July 1994 and is taking the summer off to re-evaluate her options.
Grayling UK executive chairman Vivien Hepworth said of her departure: 'Amanda had been considering taking a career break for a while. After discussions with the agency it was amicably agreed that she would leave. She has made an enormous contribution to the agency and will be missed.'
Riddle will not be immediately replaced. At Grayling she worked in the corporate, b2b, issues management and consumer sectors. She was previously with The Communications Store.
Grayling has hired an associate director, Mark Pinnes, who will join the agency next week from Morrisons, where he is acting director of media relations.
Riddle became chief operating officer when Grayling merged with sister agency The Global Consulting Group (PRWeek, 25 June 2008).