A blind woman was left "horrified" and "distressed" when staff at Tesco in north London shouted at her to get her guide dog out.
Maya Makri, 39, was in the Swiss Cottage shop 10 days after moving to the capital when she was told "never come back".
She said three cashiers shouted "no pets allowed" and to leave the shop.
Tesco said: "This clearly should never have happened and we will contact Ms Makri directly to apologise."
During the incident on Monday, Ms Makri said her guide dog of six years, Jemma, was wearing a high-visibility jacket and harness which is labelled "guide dog".
"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 said.
"I said it's a guide dog and I'm registered blind, but his two other cashiers joined in the shouting match. It was very upsetting in terms of the language and the tone.
"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."
Analysis: Damon Rose, BBC Ouch
Being asked to leave a shop or restaurant is not an unfamiliar event for assistance dog owners, though it is less common now we are almost 20 years into having had rights legislation in the form of the Disability Discrimination Act of 1995, now embodied in the Equality Act 2010.
Being singled-out and told you are not welcome is understandably upsetting to many blind or disabled people.
Depending on the establishment, however, usually it is due to poor training given to a staff member - it would be surprising if it were company policy.
If taken to court, however, that company is still likely to be liable, though pay-outs always tend to be very small around disability discrimination, barely reaching three figures.
Ms Makri, who is studying for a Masters degree at London's Central School of Speech and Drama, said she had specifically chosen the store because she could access it without having to cross the road.
Guide dogs 'allowed'
She said: "I feel so outraged because I really thought London would be streets ahead and would maybe be a place where there would be room to be treated equally.
"Tesco specifically, have the funds to provide training, and this is unacceptable."
Ms Makri said while she had been contacted by customer services and offered a £20 voucher, no one from the Swiss Cottage store had contacted her and she was still awaiting a response from the company's equality department.
Tesco added: "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."