The department store chose South African Lauren Moller to model this summer's collection, which will be displayed in all of its 26 branches from mid April, as well as featuring in the store's customer magazine Source.
The retailer said that 80 per cent of its sales of own-brand swimwear are size 12 or above and that the publicity photos should reflect this.
John Lewis spokesman Mark Forsythe said: 'This is the first time we have set out to use a size 12 model. For us, it is about having a diversity of body shapes. It is about reflecting the reality of who our customers are.'
Forsythe added that John Lewis swimwear is designed to flatter figures, with slimming panels and figure-enhancing properties, so there is 'no point' in using smaller models.
According to Forsythe, John Lewis has 'generous' size 10 mannequins in stores, but the company is considering introducing size 12 mannequins to the range.