The uthor met Lewis who played Neville Longbottom in the Harry Potter series 14 years ago at the tender age of 12.
Rowling, 49, tweeted her reaction to seeing the actor's bare frame - essentially only clad with only a pair of skintight Armani briefs and a hilarious and slightly embarrassing Twitter exchange followed.
The author, making a reference to Harry Potter star Daniel Radcliffe's full-on nudity for his theatre stint in Equus in 2008, made a light-hearted dig at Lewis.
She tweeted: '@Mattdavelewis Not as bad as watching Dan in Equus, but close. Warn me next time, for God's sake.'
An equally horrified Lewis replied, with a downcast emoticon: 'Well, this is awkward. Sorry, Jo…'
Rowling, in a very maternal manner, replied: '@Mattdavelewis I will always support you whatever you want to do, Matthew. Now go put some clothes on.'
In later tweets, Rowling revealed that she basically couldn't bring herself to even look at the images properly.
Actor Jason Isaacs, 51, who played Lucius Malfoy in the film franchise, decided to add fuel to the fire and tweeted the author, making a pun on Lewis' onscreen name.
See tweets below:
More photos below:
