The store has pledged to keep prices on a par with conventional bananas in all tiers, including value and organic, sparking speculation the move would cost £4m in subsidies.
Sainsbury's currently charges about 4p per fruit more for Fairtrade bananas, which cost 18p each.
But Tim Kershaw, chief executive of supply chain consultancy Libra Europe, doubted the chain would take a hit, because it was paying for bananas in dollars, which have weakened against the pound.
Sterling is now 11 cents higher against the dollar than at the start of October.
Kershaw also thought Sainsbury's move would attract greater numbers of ethical shoppers and could act as a boost for store footfall.
A spokeswoman for Sainsbury's refused to put a figure on the lost profit, but insisted there would be a negative impact.