The news will boost the cable company's position as the main challenger to digital satellite service BSkyB, which has 5.5m customers. Coupled with rival cable service Telewest's 564,000 customers, 1.5m people subscribe to digital cable.
ITV Digital has 1.135m customers, and forecasts that it will have signed up 1.7m customers by 2004.
NTL said it is on track to meet its year-end target of 1.25m installed customers and is signing up customers at a faster rate than its digital terrestrial rival.
The announcement will be welcomed by the government, which wants to switch off the analogue signal by 2010.
It should also help allay investors' concerns about the company's burgeoning £10.9bn debt mountain, which has caused its shares to slide recently.
NTL's managing director Stephen Carter has also announced plans to shed 5,000 jobs. Operational performance showed signs of improving when the operator announced second-quarter results last month with average revenue per subscriber growing to £39.36 a month.
NTL's broadband cable network passes around 11m homes, covering nearly 50% of the UK.