Rachel Riley left red-faced as her dress bursts open on Countdown

COUNTDOWN's Rachel Riley was left red faced after her tight blue dress burst open at the back during Monday's show.

The blonde maths whizz was placing letters on the board when the frock split.

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The bright blue mini dress featured a cut out design which appeared to break open at the zip.

Bemused viewers of the long-running Channel Four show were quick to take to Twitter about the wardrobe slip.

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One wrote: "What's going on with the back of Rachel's dress?"

Another added: "Wardrobe malfunction for Rachel Riley or her dress is one size too small for her."

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The glamorous boffin's skimpy outfits have won her many admirers.

Last month, viewers flooded YouTube with clips of her best dressed moments.

One YouTube compilation is titled “Rachel Riley – Top 5 Shortest Dresses Ever” and another shows her attempting to play mini-golf onset wearing a very skimpy frock.

It comes after a Countdown clip from 2015 showing Rachel bra-less in a tight, pink dress went viral.

The video has since been circulating online since it resurfaced and has scored around 285,000 hits on YouTube.

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Rachel has been a huge hit with fans of the show ever since she took over the role from longtime host Carol Vorderman in 2009.

She has won over viewers with her stunning dresses as well as her super-fast mental arithmetic.