NTL, the biggest of the cable giants with 3.1m subscribers compared with Telewest's 1.8m subscribers, paid $6.5bn (£3.4m) for rival Telewest.
The newly merged company, worth an estimated £6bn, will be the second largest communications company after BSkyB, which reaches 7.8m homes in the UK. Its cable footprint will cover more than 50% of UK households.
It will also be the UK's leading triple-play service provider offering TV, broadband and telephone services.
NTL chief executive Simon Duffy will head the company with NTL chairman James Mooney, while Telewest chief executive Barry Elson is set to leave.
Telewest chairman Cob Stenham is expected to stay on as a director on the new board with two other Telewest directors.
"Together, we will develop next generation services, while maintaining the ongoing improvements to existing products and customer services. We'll be better positioned to meet the future needs of our customers," Telewest said.
The merger is likely to go through with few regulatory difficulties because there is little or no geographical overlap in the two services.
An NTL spokesman confirmed branding of the newly merged company will be decided upon after the deal has been passed by regulators.
Meanwhile, Telewest's sale of its £1bn television business Flextech could be back on again after weekend reports suggest a possible sale after the NTL merger.
BSkyB was the frontrunner in the race for Flextech two weeks ago, offering over £1bn for the business, but Telewest appeared to backtrack on the proposed sale.
The decision not to sell the business to Sky was understood to be down to the satellite broadcaster's positioning as a fierce rival to cable in the pay-TV market.
Other companies still in the running for Flextech include German broadcaster RTL, which owns terrestrial broadcaster Five, and three US firms -- Time Warner, Discovery Communications and Viacom.
Flextech owns channels Living TV, Bravo, Challenge and Trouble, which feature programmes such as the top-rated 'Alias' and 'Will & Grace', and a 50% stake in nine UKTV channels with the BBC.
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