Advertisers will be allowed to plan and buy national campaigns across the cable network in a deal which could be extended to broadcast channels and is expected to be worth tens of millions of pounds to Telewest.
Further areas of partnership for the BSkyB rivals could include the purchasing of set-top boxes and programming. The companies already have a joint pay-per-view film service, Front Row, set up to rival Sky Box Office.
Advertising on NTL will be offered through Telewest's Flextech Sales from the beginning of July. Telewest already has an established sales function tied to its joint venture with the BBC, which runs six channels including UK Gold, Play UK and UK Drama.
The two cable companies have a combined total of 4.5m customers compared with 5.3m subscribers claimed by satellite broadcaster BSkyB.