The Opera singer has taken to Twitter to issue a typically dignified response to Moir, who accused her of running the London Marathon on Sunday in full make-up, branding her "fame hungry" and vain.
Despite acknowledging that Katherine had run the 26 miles to raise money for Macmillan Cancer Support in memory of her late father, Moir wrote: "No matter what Katherine does or says, however good the cause or how sincere her intent might be, she can never quite shake herself free from the thick fog of self-interest that seems to cloak everything she does."
She noted that she believed Katherine had run in "lashings of pink lip gloss, sooty false eyelashes and sweeping, coal black eyeliner" and added that "every public appearance is clearly an opportunity to sell herself".
Real nice, huh?
Celebs run the London Marathon
Hitting back at the piece, Katherine challenged Moir - who also recently slammed Claudia Winkleman in her column - to run the marathon herself.
"Jan (Moir), I wish you love & happiness in your life. I adore & support other women and I only wish you could do the same," she wrote.
"I ran on Sunday in memory of my father & to raise money (£25,000) for an excellent charity (@macmillancancer) who helped him when he was dying. Yes, I twittered about it but I did so to share my progress & day with those kind people on twitter who had supported & sponsored me.
"I ran in sunglasses because it was sunny. I tied my hair back in a pony tail because I expected to sweat. As if you had some insider knowledge you wrote I was wearing eye shadow, eye liner& lip gloss. Wrong again - none of the above - I had Vaseline on my lips, handed to us by St Johns Ambulance on our way round the route."
She added: "You can knock me all you like but you cannot take away from me what I know I accomplished last weekend. Next time you run a marathon Jan, I will be the first to sponsor you, because I know how much guts, hard work & determination it takes.
Hear, hear, Katherine! It's about time someone put her in her place.