"People have been coming up to me and shaking my hand offering their congratulations all week," he told the Mail's Richard Kay. "It's been weird. I've had to explain to everyone that I am not going to be a godfather. They have got it all wrong.
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"I spoke to William yesterday about it and he confirmed I am not going to be one. And I want to set the record straight."
Guy continued, "William wasn't annoyed about people getting it wrong, just quite matter-of-fact that it should be put right."
Guy is one of the most colourful characters in William's inner circle – and many had thought he would make the perfect modern godfather to Prince George, in keeping with William's down-to-earth approach to fatherhood.
He has insisted that his "party animal" reputation has nothing to do with the godparent decision.
"I love babies and I have calmed down quite a bit socially now. I think I will be a very, very good godfather – one day at least. But it is their choice."
It remains to be seen who William and Kate will choose to join them in the Music Room at Buckingham Palace next month for the christening of their son.
It is thought that six godparents will be asked to stand by the royal couple's side as Prince George is baptised with holy water from the River Jordan.
Among those tipped for the honour are Pippa Middleton, Prince Harry, William's best friend Thomas van Straubenzee, former private secretary Jamie Lowther-Pinkerton, ex-nanny Tiggy Pettifer née Legge-Bourke, and Kate's Marlborough friend Alicia Fox-Pitt.
Hugh van Cutsem - whose father recently passed away - is also in the running. His daughter Grace was a bridesmaid when the Duke and Duchess tied the knot, and the little girl is also William's goddaughter, making it all the more likely he'll return the favour.
Who do think would make an ideal godparent for the royal baby? Leave your comments below.