The resignation email, sent this morning (25 September) by Kieran Allen, a senior account manager at MEC, listed a series of grievances against his manager.
The email was quickly disseminated around rival agencies and details of it trended on Twitter. A full transcript of the email was also published on Twitter by Loaded Magazine @loadedmag.
In a statement the agency, which is led by chief executive Steve Hatch, said: "We are sad that one of our employees has chosen to share their personal views in such a public way and has left the company with such bad feeling. We are taking this issue seriously though given the highly personal nature of the email, we cannot comment further."