Yell has ended its 23-year relationship with Abbott Mead Vickers BBDO by putting its £20 million advertising account up for review.
The business is being moved owing to conflict with AMV's BT client, which is entering the classified directory market.
Ann Francke, Yell's chief marketing officer, will handle the pitch. It is understood that concern about conflict comes from Yell, not BT.
Proximity, AMV's sister direct marketing agency, which won Yell's direct marketing business two years ago, is not affected by the review at present.
However, it is understood that this may change if the agency wins the pitch for BT's 21CN account, BT's planned internet protocol-based network, which aims to change how telephony is delivered into homes.
Proximity already works on a small part of BT's DM account, but this win would make it a major player on BT's direct marketing roster.
Yell was originally part of BT's classified directory service, but the telecoms company sold the business in 2001 to Apax Partners and Hicks Muse Tate & Furst. At the end of last year, BT announced its intention to launch its own directory into the market in which Yell has an 84 per cent share.
AMV said in a statement: "BT's recent move into the classified directory arena has meant that it has become an important direct competitor to Yell. So it is with enormous regret that Yell and AMV have decided to end their long relationship."
A spokesman from Yell said: "The 23-year relationship Yell has enjoyed with Abbott Mead Vickers has been a joyous and successful one. Those at the agency who have worked with us over the years remain our favourite people in advertising."
During its 23 years on the Yell, formerly Yellow Pages, account, AMV created classic work for the client, including the famous TV spot about "JR Hartley", an elderly author who uses the Yellow Pages to find a copy of his out-of-print book Fly Fishing.
The recent TV ads feature James Nesbitt, the star of the TV series Cold Feet, using the company's directory services to order services ranging from dog trainers to fashion stylists.