Judy Murray, 59, told her 248,000 Twitter followers she went to pay for two donuts in the capital when her Scottish money was refused.
The mum of Wimbledon champion Andy Murray, 31, uploaded a picture of the scrumptious looking treat but complained about the service.
She wrote on Twitter: “When you go to pay £9 for two donuts (yes, really) in London and your £10 Bank of Scotland note is refused because, ‘We only take British ones’.”
Many of her thousands of followers commented to reveal their own similar stories, whilst others also expressed their outrage.
“But they’ll accept your Scottish son to play for Britain,” said one person.
Another said: “That’s ridiculous! I would have said ‘You’re the great @JudyMurray, you can have the two donuts for nothing!’”
A third added: “Oh man is this still happening? I normally just take my money somewhere else.”
Scottish banknotes are currently legal in the UK but they are not legal tender.
This means the currency can be used if a business wants to accept it but there is no legal requirement for them to do so.
Scottish MP, Liberal Democrat deputy leader, Alistair Carmichael, is trying to change the law to make the currency legal tender.
He’s lodged a bill in the House of Commons and said: “It’s beyond time we give legal force to the well-known phrase, ‘That’s legal tender, pal’.”
Meanwhile, Judy spoke about Andy’s future in tennis earlier this month after he underwent hip surgery.
The mother-of-three said she was “hopeful” about his return but said it’s “early days”.
Judy told Scotsman: “Hopefully he does come back and play again but you never know.
“He’s doing OK, but it’s early days and we’ll just have to see how things turn out.”
In January, Andy uploaded a picture of himself following his surgery where he appeared in good spirits.
He captioned a snap of himself lying in a hospital bed: “I underwent a hip resurfacing surgery in London yesterday morning.
“Feeling a bit battered and bruised just now but hopefully that will be the end of my hip pain.”
He added: “I now have a metal hip as you can see in the 2nd photo and I look like I’ve got a bit of a gut in photo 1.”