A blind woman shopping at Tesco was ordered to leave by three "shouting" staff adamant she could not bring her guide dog into the store.
Maya Makri, 39, said she was in a branch in Swiss Cottage, north-west London, when the cashiers told her pets were not allowed in the store and told her to leave.
Her guide dog Jemma was wearing a high-visibility jacket and a harness labelled "guide dog" when the incident happened on Monday.
"I got to the till and could sense there was an uncomfortable atmosphere then the man behind the till started shouting 'no pets allowed'," she told the BBC.
"I said 'it's a guide dog and I'm registered blind', but his two other cashiers joined in the shouting match.
"Everyone in the queue kept saying 'she's allowed to have a dog' but they (cashiers) were just completely adamant that dogs were not allowed in the store."
Makri, who moved to London from Leeds to study, said the exchange was "very upsetting" and left her feeling "outraged".
Later she wrote on Twitter: "To clarify, I blame Tesco, a multinational worth billions for not putting proper training in place, not the staff that are on minimum wage."
She also said it was not the only instance in which she had been refused entry because of her guide dog.
"Everywhere I turn I'm being refused access with my guide dog since I've moved down to London," she wrote. "What's wrong with you people?"
Tesco stressed guide dogs are allowed in stores.
A spokesman said: "This clearly should never have happened and we will contact Makri directly to apologise.
"We do allow guide dogs in stores and have reminded colleagues of that.
"We also offer customers with guide dogs help with their shopping, if they would like assistance."