Interflora won a silver gilt medal for a display that included just one British-grown variety out of 60 after saying its concept would "represent the Best of British [and] incorporate British grown flowers" such as roses, peonies, aliums and stocks. Only the stocks, grown by Sue Lamb and supplied by Butters, were British.
Chrysanthemums Direct exhibition manager Martyn Flint, who also won silver gilt for his British-grown display, said: "The origin of the plant material has no consequence in RHS judging. A lot of exhibitors feel if you grow the plants yourself it should put you have someone who buys in from abroad."
The RHS said it regularly assessed judging criteria.
RHS director general Sue Biggs said: "We will mention the fact to Interflora in the show wash-up but it is not our role to control them."
She said the RHS was promoting UK growers at a Wisley British specialist nursery area.
HTA chief executive Carol Paris said: "We don’t want someone to suggest something is British when it isn’t. If something is advertised as British and it isn’t there needs to be a mechanism in place to correct that so it is not confusing the consumer."
The ‘Britwashing’ echoes an Interflora/RHS Valentine’s Day bouquet in 2014, promoted as 60 per cent British, which contained just two British flower varieties.
Stand designer David Ragg said to HW in the show build-up: "We tried to incorporate English flowers where possible but the weather was against us this year."
However, British grown cut flowers campaigner Gill Hodgson said: "I know they could have got British varieties."
Interflora said after the show: "It was never intended to showcase only British grown flowers."
The RHS/BBC dropped a Best of British theme in the show marquee, and the NFU/Waitrose pulled out of the UK Horticulture exhibit at the show this year.