A national postal strike could cost retailers £840 a week as they are forced to seek alternative, often more expensive, delivery companies.
The strike, expected to go ahead next week, could cost the economy £1.5bn.
Retailers’ costs could hit £270m according to think tank Centre for Economics and Business Research.
Online sales have continued to thrive through the downturn but it is feared a strike will have a devastating impact for the home shopping sector in the crucial run up to Christmas.
The BRC has called the strike “foolish” as it is damaging one of its best growth areas, online retail.