John Lewis sought out curvy South African Lauren Moller to be photographed wearing its 2007 collection.
The fashion chain said 80 per cent of its own-brand swimwear sales were in sizes 12 and above.
It previously used a size 10 model for swimwear publicity but has chosen a model reflecting customers' "real-life body shape".
John Lewis chose size 12 5ft 8in Ms Moller from Cape Town who has a 26in waist, 34C bust, 37.5in hips and a body mass index (BMI) of 19.8.
John Lewis public relations manager Mark Forsyth said modelling agencies contacted by the chain said size 12 was considered a "plus size".
He said clothing sizes still varied between UK stores.
"It is very difficult to quantify exactly what a size 12 is. It shows you the kind of tyranny of trying to fit into one size group because it doesn't really mean very much as they're so different for different brands," he said.
The new swimwear pictures will appear in John Lewis stores and in other publicity material.
The size zero debate hit the headlines during last week's London Fashion Week after organisers decided not to ban models with a BMI of under 18 from the catwalk.
Instead, the British Fashion Council asked designers to use only healthy-looking models aged over 16.
A US size zero – equivalent to a UK size four – last year became the target size for a number of high-profile female celebrities in the US.