The dispute is said to relate to differing valuations of the five audience. Carat has been involved in similar stand-offs in the past, but has managed to negotiate through them.
There is an added twist to the disagreement because five is a client of Vizeum, which has handled its media planning and buying since 2003.
Sources said a smaller spat, involving ZenithOptimedia and ITV's digital channels, has yet to be resolved, with ZenithOptimedia yet to commit to its deal with channels including ITV2 and ITV3.
Meanwhile, Sky Media has emerged as the broadcaster to lose ground during the TV trading season.
Sky is expected to lose around 0.5 per cent of its share of total TV revenues, according to agency sources.
Sky's decline follows the row with the cable operator Virgin Media, which led to Sky channels being pulled from the Virgin platform.
Sources said that ITV's share of the £3.5 billion TV market had remained flat after a relatively good 2007 under the terms of the CRR trading mechanism, and that five may also lose some share. Agencies indicated that, after a couple of tough trading seasons, Channel 4 had increased its share slightly, by 0.2 per cent.
One agency broadcast director said: "ITV went into negotiations with a policy of 100 per cent retention under CRR and there's been little deviation from that. ITV's determination has dictated the tone of the negotiations round."