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 the store's customer magazine Source.
The retailer said that 80 per cent of its own-brand swimwear sales are a size 12 and upwards 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. We were very surprised - we were referred to the plus-size agencies and departments. When we went back to the agencies and confirmed that it was not plus-size, but size 12 models that we were after, the agencies confirmed that in their world, size 12 is considered a plus size.'
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.
He said: 'For us, it is about having a diversity of body shapes. It is about reflecting the reality of who our customers are.'
According to Forsythe, John Lewis has 'generous' size 10 mannequins in stores, but there is talk of introducing size 12 mannequins to the range.