Scotland may have received help from an unexpected corner in its dramatic win against Italy in the Six Nations on Saturday: JK Rowling has revealed in a new story why "it is considered infra dig for wizards to support any rugby team other than Scotland".

Ever since the 19th century, writes Rowling on her website Pottermore, the worldwide wizarding community has thrown its support behind the Scottish rugby team, even though they are forbidden to take part in "Muggle" sports themselves. In a rare appearance on Twitter, Rowling also urged her nearly three million followers to "help keep this noble tradition alive by tweeting #wizards4scotlandrugbyteam before #6nations Scotland v Italy", adding: "Wizards worldwide support the Scottish rugby team. It's an old magical tradition."

"The wizarding world's affection for the Scottish rugby team is all the more bizarre because a substantial part of wizarding society knows nothing about Muggle sports, which they regard as inherently dull and even silly," she writes in her new story on Pottermore. "Yet the Scottish rugby team has become a wizarding meme – part in-joke, part genuine interest – which has its roots in the 19th century and is a tale both sad and uplifting."

Cricket, on the other hand, is widely ridiculed by wizards. "As the chief sports writer in the Daily Prophet wrote in 1902: 'a Beater who is unable to fly defends three sticks instead of a hoop, while a Snitch without wings is thrown at the sticks. That's it. Sometimes for several days'. Rugby held more appeal," writes Rowling. "Wizards could not help but admire the strength and courage of Muggles prepared to engage in a sport so brutal, without recourse to Disapparating out of the way, or access to Skele-Gro to repair broken bones. It must be admitted that there was an edge of sadism to some wizards' enjoyment."

Her story tells of the "squib" – "a wizard-born child with no magical powers" – Angus Buchanan, born in the mid-19th century, who was cast out at the age of 11 by his family. Large, strong and fast, he took up rugby, and ended up playing for Scotland in the first ever international rugby match, Rowling writes. After he published the worldwide bestseller My Life as a Squib, he "became world-famous among wizards whilst also being celebrated among Muggles, a hitherto unknown achievement", and "wizards of many nationalities began turning up to watch him play sport".

Today, "discussing Scottish rugby has become one of several covert identifiers for wizards meeting in front of Muggles and seeking to establish each other's credentials", while the Wizarding Supporters of Scottish Rugby Union "meet on the eve of every Scottish international match to toast Angus's memory and anticipate a happy 80 minutes of watching Muggles trample each other into the mud".

"The International Statute of Secrecy expressly forbids wizards to participate in Muggle sport, but there is nothing illegal in supporting a Muggle side," writes Rowling. "However, the WSSRU has often had to deny the persistent rumour that its secret mission is to smuggle a talented Squib on to every Scottish team. Current suspects include Kelly Brown (possible cousin of Lavender's), Jim Hamilton (strong resemblance to Hagrid) and Stuart Hogg (enough said)."

The magical support clearly worked: Scotland snatched victory from Italy with a last-minute drop goal from Duncan Weir, winning the match 20-21 on Saturday.