It will initially press all the London mayoral candidates to support the plans as a platform to build popular support.
Luther Pendragon prevailed in a three-way pitch to land the business. Senior consultant Susie Winter heads the account.
SLP proposes building on the existing tram link from Wimbledon to Beckenham via East Croydon to take in four more routes: Streatham to Coulsdon via West Croydon, Sutton to Wimbledon, Sutton to Tooting and
South Norwood to Crystal Palace.