She's known for her speedy maths skills and co-presenting duties on Countdown.

But Rachel Riley, 36, had to hold back her laughter during a recent episode of the game show, as the board spelled out an X-rated word.

She had been forced to rearrange a mix of vowels and consonants to spell 'Bonking' at a contestant's request, before giving a smirk to the camera.

One of the show's contestants, Joe, formed the word from the selection of letters - which were B, E, L, N, O, I, K, N and G - Rachel in turn had to create the word 'Bonking' on the board.

Guest host, Reverend Richard Coles was left flabbergasted and asked what it was. 

But lexicographer Susie Dent insisted it was a recognised word, saying: 'It's in the dictionary, there are lots of different meanings though.

'You can bonk someone on the head, or if you're a cyclist and you hit the point of exhaustion - which I've done often - you bonk as well.'

It's certainly not the first time Rachel, who has presented the show for over a decade, has had to keep her cool at the board.

Only last month, she had to arrange the mix of vowels and consonants to spell 'Jiz Louder' on the board.

The show's contestants, Tom and Steve, attempted to form a word from the selection of letters - which were I, D, L, O, R, U, E, J, Z.

Tom suggested 'louder', and as Rachel rearranged the letters to their correct form and created the six letter word, - she in turn had to create the phrase 'Jiz Louder' on the board.

'Vaginas UK' was also spelled out back in July.

And the phrase wasn't so overlooked that time, with contestants Thomas and Mike even seeing the funny side.

The final set of letters was I, N, V, A, A, G, S, U and K and presenter Colin Murray had asked Thomas what his word after the clock ticked down.

He said: 'vagina' but when Colin asked Mike what his word was, he replied 'vaginas.'

Colin, clearly tickled by the funny moment, said: 'Vaginas, well, there you go, putting an s on the end, what a difference that makes.

'Always pluralise. Pluralise everything, and there you go.'

The moment left audience members in stitches and they laughed along, with Rachel raising an eyebrow.