Farm, which has held the £1.5 million creative account for three years, lost the brief to AMV without a pitch.
The £5 million DM account, which has been with Rapier since 2003, also went to Claydon Heeley without a pitch.
The Smart car brand, which launched as a joint venture between Mercedes and the designers behind Swatch in 1993, recorded a 29 per cent fall in UK sales last year.
The marque has recently undergone a strategic rethink and axed its roadster sports car and is phasing out the four-door forfour model.
Meanwhile, DaimlerChrysler's flagship Mercedes brand is also considering a consolidation into AMV. The £13 million UK business is expected to move from Shop at the end of this year.
AMV, Claydon Heeley and Smart did not return calls.