McCann has started telling clients including Bacardi and Cereal Partners, but no further details were available as Campaign went to press.
Hunton, 43, joined McCann in 1999 as its deputy managing director. He rose through the senior management line-up during his four-year tenure and was promoted to chief executive of the London agency by McCann's then European chairman and chief executive, Ben Langdon, in January 2003. He replaced Tamara Ingram, who Langdon ousted.
During his time at McCann, Hunton was credited with developing strong relationships with a number of key clients including Mastercard and Bacardi.
News of Hunton's departure comes less than a year after Rupert Howell, the UK and Ireland chairman, took the helm of McCann with a brief to reinvigorate the agency.
Howell was unavailable for comment.