The two businesses, which will be merged under the Virgin brand name, confirmed this week that agency talks were on the agenda.
A spokesman for NTL/Virgin Mobile said: "A vigorous, objective evaluation process will take place to ensure the company has the agencies it needs to ensure consumers reap the full benefit of the merger."
Agencies on NTL's roster include Rapier, AKQA, Kinetic Worldwide and WWAV Rapp Collins, while the Virgin Mobile roster includes Rainey Kelly Campbell Roalfe/ Y&R, Manning Gottlieb OMD and 141 Worldwide.
RKCR/Y&R, which has held the Virgin Mobile account since 1999, recently won BT's business-to-business account, which is a direct competitor to NTL. This is likely to reduce the agency's chances of holding the merged account.
The deal between the two companies, agreed this week, creates the UK's first quadruple-play platform, offering customers the chance to take up television, internet, land-line telephone and mobile telephony from one supplier.
This will put the company in direct competition with BSkyB and BT, which both offer triple-play platforms.
The deal is expected to accelerate the take-up of video-mobile technology. The new company will be aggressively bidding for Premiership rights in an attack on BSkyB as it looks to set up a Virgin-branded sport channel.
The spokesman added: "We strongly believe that offering quad-play underpins true convergence in the UK."
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