WCRS has been given the task of producing a commercial to promote 3's range of pre-pay services.
The appointment adds to the speculation that WCRS is on the brink of wresting the entire creative account from TBWA once the agency's senior management completes its buy-out from its parent, Havas Advertising.
The agency, which has until now only worked in a strategic capacity for 3, was called in at short notice to handle the task and shot the spot earlier this week in New York.
A 3 spokeswoman claimed the commission would not affect TBWA's hold on the main creative account. She said: "TBWA is our above-the-line agency.
It is responsible for our creative work. WCRS has a very strategic brief." However, she did not rule out the possibility that WCRS would handle more advertising for the company in the future.
The new ad will maintain the positioning and creative theme used in TBWA's latest campaign, which features the actress Anna Friel.
WCRS has close ties with senior management at 3 and was involved in secret negotiations with it before it held a full advertising review in June 2003, which resulted in TBWA being reappointed. WCRS has substantial mobile network experience: it launched Orange in the UK and has also handled the UK Vodafone business.
Details of WCRS's management buy-out remain closely guarded but it is understood an agreement is imminent and that the agency will retain its name and Golden Square HQ.