A deal due to be signed this weekend will see Nitro take what is being described as a "substantial" shareholding in the one-time Bartle Bogle Hegarty breakaway.
The operation, provisionally called Nitro/Soul, will start trading at the end of the month. It brings together Soul's £12m-worth of business, including Fanta and Tresemme, with Nitro's global assignments for Mars, Unilever and Swatch.
The tie-up reunites Paul Shearer, the former Leo Burnett executive creative director, with Soul executives who worked with him at BBH. Shearer and Clarke will both join Soul's board.
Meanwhile, it is believed that another deal is in the offing which will give Soul extra critical mass.
"We will be looking to acquire in the UK to broaden the offering," Bruce Crouch, Soul's creative partner, said.
News of the deal culminates a tough trading period for the 70th-ranked Soul which was hit by the loss of its £8m Harvey's furniture business and the Aunt Bessie's food brand last year. "This gives us access to international opportunities we've not had before," Crouch added. "It enables us to step up a gear."
For Nitro, founded in China in 2001, the agreement with Soul adds a UK presence to offices in Melbourne, Sydney, Shanghai, Hong Kong, Taipei, and New York.
Soul was formed five years ago by the former BBH colleagues Crouch, Duncan Bird, Andy Bird and Seamus O'Farrell, and Kevin Brown from sister media agency Motive.
Clarke, who opened talks with the founders at the end of 2004, praised Soul's "impeccable pedigree" and said the new agency would spearhead Nitro's move into Europe. "This is one of those deals where everyone benefits."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .