Blue State Digital is developing campaign elements that it hopes will help persuade the government to reverse its plans to partially privatise the public company.
A spokesman for the CWU confirmed that the agency have been tasked to use tools that will "drive the protest message quickly and effectively".
The agency, which is hoping to woo the Labour Party and win its digital business, will be running online petitions and developing a social networking strategy.
The CWU is also looking to build its email database.
Blue State Digital's appointment comes as another player enters the bid to buy a 30% stake in the beleaguered publicly owned postal company.
De Post La Poste, a Belgian private equity backed postal group, will now make a bid alongside buyout firm CVC Capital Partners.
Dutch postal company TNT and DHL, a subsidiary of Deutsche Post, are also in the running for the 30 per cent share.
The government's plans, which are being overseen by business secretary Lord Mandelson, to sell the stake off to the private sector have been met with heavy criticism.
Workers, trade unions as many as 100 Labour backbenchers have all condemned the move.
Yesterday, in an interview with The Observer, business secretary Lord Mandelson accused the CWU of using "scare tactics" that could cost workers their pensions and their jobs. Mandelson said that the strength of opposition will not prompt the government to back-track on its sell-off plans.