JWT has slipped from second spot in the agency billings league for the first time, according to Nielsen's most recently published figures.
The agency, which lost both its Vodafone and Reckitt Benckiser accounts in the past 12 months, fell to third place in August after seeing its billings drop £64 million to £285 million since December 2006.
McCann Erickson has leapfrogged the WPP agency to take second place by increasing billings to £305 million - a growth of 4.5 per cent since the end of last year.
Other agencies to record growth include Bartle Bogle Hegarty, which shot up from seventh to fourth after picking up Vodafone and Procter & Gamble brands. The agency increased billings by 27 per cent, from £212 million to £270 million, over the period.
WCRS has moved in to the top ten for the first time after increasing billings by 55 per cent, from £134 million to £208 million, on the back of wins including Sky and Littlewoods. The agency jumped from 17th to ninth.
Will Orr, the managing director at WCRS, said: "Our priority is great strategy and creative work. Our recent growth is a welcome sign that we're delivering on that."
Fallon has broken into the top 20 for the first time, moving from 24th to 19th. Last year's Campaign Agency of the Year increased billings from £60 million in December to £113 million in August, representing growth of 88 per cent.
August's data also reveals that 2007 has been a challenging year for many network agencies. DDB London fell from ninth to 11th place over the period, while Publicis dropped from fifth to eighth after losing Asda and MFI.
Ogilvy Advertising fell from eighth to tenth; Leo Burnett from tenth to 12th; Saatchi & Saatchi from 11th to 14th and Grey from 13th to 15th.