Knapp founded NTL as CableTel in 1993 and oversaw its transformation to become the biggest UK cable operator through a series of acquisitions, including that of Cable & Wireless Communications.
At the end of last year, with debts in excess of £12bn, Knapp led the company through a financial restructure, which saw it convert £7bn of debt into equity and saw the company split into two divisions -- NTL UK and Ireland, and NTL Europe. It also filed for Chapter 11 bankruptcy status, from which it emerged from last quarter.
Knapp announced his resignation at the group's second-quarter results, where the company reported narrowed losses down to £159m from £284.8m last time on revenues of £551.3m, up from £549m a year ago.
"With this quarter's positive results and the favourable conditions I see ahead for NTL, I believe my mission in bringing NTL through its reorganisation has been accomplished," Knapp said.
"Further, in Simon Duffy the company has a strong new leader who is clearly up to the challenges. I'm stepping down as CEO at close of business on August 15, knowing that the company is well positioned and in very good hands," he added.
NTL saw monthly average revenue per user rise by 4% on last quarter to £41.04, while churn was down year on year to 12.9% from 17.1% last time.
The company said it added 129,000 customers in the second quarter across all services, bringing the number of customers to 2.75m, with 1.3m of those being digital TV customers.
Knapp relinquishes his duties as CEO on Friday August 15, but will continue working as a consultant to the business until the end of the year.
Duffy joined the company as chief operating officer six months ago from Orange, owned by France Telecom, which has a 25% stake in NTL.
"NTL is a great company with enormous untapped potential. I intend to ensure that we deliver it," Duffy said.
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