Duffy will be replaced by Simon Burch, a veteran cable television executive from Comcast, the US media group.
The move comes despite NTL being in the middle of two major takeover bids, one for cable operator Telewest and the other for telecoms group Virgin Mobile.
Duffy's new role will focus on group strategy and business development, regulatory affairs and mergers and acquisitions.
Burch was responsible for Comcast's acquisition of AT&T Broadband which brought in two million new subscribers. He will take on his new role at NTL in the new year.
