According to the latest figures from Nielsen Media Research, MediaCom held its lead over ZenithOptimedia after overtaking its rival at the end of 2003.
MediaCom's billings growth matched the total growth among the top 30 agencies, also at 5%. Total spend through the top 30 buying points was up by £306m to £6.4bn.
ZenithOptimedia, which balanced account losses such as KFC by winning O2, suffered an overall drop in its billings, while MediaCom puts its growth down to a strong new-business performance and a good client-retention record.
Stephen Allan, the chief executive of MediaCom, said: "Growth has come, of course, from new business with big wins, such as Boots and Churchill, starting to feed through. But we've also seen an upturn in organic spend with existing clients spending a bit more money."
MindShare leapt ahead of Carat to become the third-largest agency after winning accounts including Abbey and Argos.
The fastest-growing agency in the top 10 was Initiative, which picked up the France Telecom account and additional business from General Motors towards the end of last year.
Manning Gottlieb OMD leapfrogged its sister agency PHD to enter the top 10 at number 10. PHD's 15% drop in billings came about as a result of losses including Weetabix and Homebase. Its buying operation is now consolidated with Manning Gott-lieb's as part of the OPera buying group.
Outside of the top 10, a group of independent agencies posted significant growth, suggesting there is life beyond the networks. Walker Media led this wave, recording a 22% billings increase on the back of wins including KFC and healthy spending from clients such as Dixons.
MediaVest Manchester, 75% of which is independently owned, grew by more than a quarter. Other independents including Brilliant Media, John Ayling & Associates and BLM Media also grew rapidly.
John Ayling, which was placed 21st, experienced a 25% increase in its billings, which reached a total of £60.2m.
BLM, the 25th-largest agency, rose two places based on a 32% rise in its billings to £50.1m.
However, independence was no guarantee of growth with The Allmond Partnership suffering billings declines following its loss of O2.
Feather Brooksbank's large growth came ahead of its recent loss of the multimillion-pound Scottish Executive account to MediaCom Scotland.
The direct agencies All Response Media and Zed Media, placed 26th and 29th, showed strong growth.
Agencies rank on billings to July 31 2004
1 MediaCom £640m 5.5%
2 ZenithOptimedia £611m -3.3%
3 MindShare £572m 11.8%
4 Carat £543m 0.6%
5 Initiative £488m 22.2%
6 OMD £460m 6.7%
7 Starcom Motive £377m -2.5%
8 Universal McCann £339m 7.1%
9 Starcom MediaVest £275m 3.4%
10 Manning Gottlieb OMD £257m 11.8%
11 PHD £236m -14.6%
12 Vizeum £213m 1.4%
13 Walker Media £188m 22%
14 Mediaedge:cia £184m 1.6%
15 MediaVest Manchester £110m 25.7%
16 Media Planning Group £109m -14.4%
17 Brilliant Media £82m 24.7%
18 Feather Brooksbank £79m 31.3%
19 Brandconnection £77m -3.7%
20 The Allmond Partnership £69m -12.6%
Source: Nielsen Media Research
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