A woman claims her gym locker was raided when she was exercising - only for the crooks to then take themselves on an £8,000 shopping spree with her cash.
Charlotte, from West London, told the BBC that she was made to feel like a “criminal” by her bank Santander - who argued that she must have shared her PIN number.
She believes the thieves were able to access her money through her personal banking app on her phone.
Santander has since apologised and refunded her money.
The incident occurred when Charlotte was exercising at Virgin Active gym in Chiswick on August 24. She says her locker had been padlocked.
She told BBC Radio 4's You and Yours programme: "I was completely stuck, I just froze."
The operator told Charlotte that in the space of 90 minutes, her card had been used to make around £8,000 worth of purchases from her current account.
Purchases had been recorded in pricey high street stores like the Apple store at Westfield shopping centre in Shepherd's Bush, the Apple store in Regent Street, and Selfridges on Oxford Street.
She then learned that the £10,000 in her savings account, which she initially thought was safe, had been transferred to her current account.
Charlotte said she felt "broken" when Santander contacted her a few days later saying that they had only managed to stop a few of the purchases, leaving her £5,000 out of pocket.
"I was very rudely and bluntly told 'you will not be getting your money back. It's your fault because they've used your PIN'," she said.
"I was also accused of writing my PIN down and keeping it on a bit of paper in my bag."
Charlotte says she found herself "frantically trying to prove [her] innocence" as she took to social media to raise awareness of her experience.
She then received a call from a senior Santander staff member, who "apologised wholeheartedly" and "held her hands up".
Santander told the BBC it has paid Charlotte £750 in compensation for the poor customer service she received and for "incorrectly declining her refund request”.
However, the bank said that their security logs show that there had been "no compromise with our mobile banking app".
A spokesperson from Santander said: “We take protecting our customers’ accounts extremely seriously and have comprehensive fraud prevention systems in place.
"We are investigating the details outlined by our customer on social media as a matter of urgency and contacted them as soon as we were alerted to the case to discuss the issue further.”
Virgin Active gym said it was assisting police with their inquiries and "fully investigating all of the circumstances around the brief failure of the access gates".
The Mirror has contacted Virgin Active for further comment.
