It is not clear whether Lee, who told PRWeek she was 'looking forward to taking on a new and exciting challenge that delivers full integration to clients', has a job to go to.
The CEO role has been taken over by Ketchum’s Europe CEO and UK chairman David Gallagher. An agency spokeswoman said he would be in the role ‘indefinitely’.
Gallagher said: 'We are grateful for Avril’s achievements during her time at Ketchum. We thank Avril for her many years of service and wish her all the best.'
Lee worked her way up to partner level at the Omnicom-owned agency, rising through its healthcare practice, and was appointed UK CEO in February 2011.
Lee said: 'I have decided to leave Ketchum having spent nearly ten years at the company. I’ve run Ketchum London successfully for many years now, most recently as CEO for two years and before that as Deputy CEO for several years.
'In this time I’ve achieved many of my professional goals and worked with world leading clients on fantastic campaigns as well as adding innovative new offers and service in London.
'It has also been great to see this recognised with numerous agency awards in the last few years.'