The maths whizz revealed she was once introduced to a male fan by his wife before he groped her backstage before the couple were banned from the audience.
Rachel said: “This married couple came up to me and the lady said ‘oh my husband really likes watching you, do you mind if we just…’ and he squeezed my a**e.
“I didn’t know what to do with myself.
“I was a bit shocked and stood back and they didn’t get invited back to the audience.”
Rachel went on to tell The Sun that “you get the odd a**e grabber” – suggesting that this was not a one-off event.
Rachel, now on maternity leave, announced her last day of filming earlier this month via Twitter to her 689,300 followers.
She wrote: “Well that’s it from me, I’ve officially finished filming to pop a baby out!”
The star went on to explain that maths genius Dr Anne-Marie Imafidon, who previously appeared on Channel 4’s “Black To Front” edition of the show, would be stepping in to fill her shoes.
“You’re stuck with me on screen till December when the totally amazing @aimafidon will be keeping the numbers in line till I’m back next year!
If, like us, you love a good dose of celebrity gossip then we’ve got you covered.
From Lorraine Kelly and Janey Godley to Gordon Ramsay and Sir Rod Stewart, our weekly Showbiz newsletter is packed with the latest news on your favourite stars from Scotland and beyond.
Signing up couldn’t be easier! Simply enter your email address in the box further up the page or click here to view our wide range of newsletters.
“Hope you have loads of fun Anne-Marie, best of luck lovely!”
Rachel is currently expecting her second baby with former Strictly professional dancer Pasha Kovalev, 41, who was her partner on the show back in 2013.
Following the presenter’s split from her first husband Jamie Gilbert, Rachel and Pasha began dating.
The couple married in Las Vegas in June 2019 and welcomed a baby girl at the end of that same year called Maven Aria.
Rachel has been the face of Countdown since 2008, when she replaced the glamorous Carol Vorderman, 60, who had previously presented the show for 26 years.
She also co-hosts 8 Out Of Ten Cats Does Countdown, a late night version of the show, with comedian Jimmy Carr, 49.
Aside from opening up on the groping she was subjected to whilst filming Countdown, Rachel has also complained about the way she has to dress on the show.
The celeb said bosses used to insist she dress like “her own mother”, which led to her fashion choices being painfully dissected on social media.
On the Channel 4 show Complaints Welcome, she explained: “I had to buy some new clothes at one stage.
“I just didn’t know what to wear and they were trying to dress me more like my mother at that stage, everything had to be below the knee.”