The former chief executive of the Lowe agency in London is believed to have become marginalised following the recent arrival of Tom Bernardin, the former Bozell chief executive, as the chief operating officer.
Bernardin, described as charming, straight-talking and very bright and said to be on the network "fast track", reported directly to Jerry Judge, the worldwide chief executive, along with Hammersley. "It was never going to work," an industry source said.
Hammersley, 41, was himself cast as the network's "golden boy" when he went to New York in March 2001 with a brief to transform the US agency's performance after a series of account losses.
During his tenure the agency lost its Heineken and Diet Coke business but picked up new assignments from Carfax, Federated Department Stores, Electrolux and Cablevision.
However, some insiders believe Hammersley's blunt style may have led to the split. "Paul is a very forthright character who doesn't necessarily operate in the way Lowe would have wanted him to," an insider said.
Hammersley said this week that he wanted to "take advantage of some new opportunities", which has led to speculation that he might consider starting his own agency.
Judge said: "We respect Paul's decision, which he has made for the most honourable reasons. He has been a highly valued colleague at Lowe for ten years and we will miss him greatly. We wish him all the best."