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.

Many retailers including John Lewis and House of Fraser have said they will seek alternatives to the Royal Mail until after strike action has been resolved.

Online fashion retailer Asos has promised its customers that deliveries will not be affected and eBay said it was seeking alternative couriers for its sellers.

The BRC has called the strike “foolish” as it is damaging one of its best growth areas, online retail.