A campaign has been launched to make Scottish currency legal tender across the UK after Judy Murray was told she couldn't buy doughnuts with it.

The 59-year-old coach - and mother of tennis superstars Andy and Jamie - tried to pay for two doughnuts at a London bakery on Sunday, but was told her Scottish currency was not valid.

When you go to pay £9 for 2 donuts (yes, really) in London and your £10 Bank of Scotland note is refused because “we only take British ones”. 🤔🤣 pic.twitter.com/SCCzIfolDn — judy murray (@JudyMurray) 8 April 2019

Murray, whose sons have won Grand Slam titles including Wimbledon, tweeted: "When you go to pay £9 for 2 donuts (yes, really) in London and your £10 Bank of Scotland note is refused because "we only take British ones".

Now a Scottish MP is trying to change the law to have Scottish banknotes accepted throughout the UK and to force businesses to accept them as payment.

View photos Scottish banknotes are legal currency in the UK but, contrary to popular belief, they are not legal tender More

Liberal Democrat deputy leader Alistair Carmichael has lodged a bill in the House of Commons, saying: "It's beyond time we give legal force to the well-known phrase 'that's legal tender, pal'".

Scottish banknotes are legal currency in the UK but, contrary to popular belief, they are not legal tender.

According to the Royal Mint, the phrase "legal tender" is a narrow technical term referring to the settlement of debts, and in ordinary transactions both parties can agree to accept "any form of payment".

In other words, Scottish notes can be used if a business accepts them, but there is no legal requirement for them to do so.

Mr Carmichael's Legal Tender (Scottish Banknotes) Bill would aim to put an end to this.

The former Scottish secretary said: "Every Scot who travels south has a story about their money being turned down or looked at sceptically. There is no reason to make a distinction between Scottish and other banknotes as acceptable payment.

"I hope that this bill will encourage businesses across the UK to recognise and accept Scottish currency."

Current Scottish secretary David Mundell proposed a similar bill a decade ago, but it did not become law.