According to a report in the Sun, the 31-year-old aristocrat will be one of six godparents.
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A source told the paper he is "thrilled to bits" and "everyone was congratulating him". "Guy's been a very discreet and good friend to William over the years. Those are qualities he wants to pass on to George."
Despite the fact that Guy has been at all the most lively parties involving William and Prince Harry over the years - he organised the stag do before the royal wedding in 2011 - details rarely leak out. That kind of loyalty is very important to the Princes, who guard their privacy as much as possible.
He once ran Mahiki, one of the royals' hotspots in central London, and usually kept an eye out for the Duchess of Cambridge at social events if William had other commitments.
Also in the running is Thomas van Straubenzee. Another constant companion of the future King's over the years, he was at school with him and accompanied him on his gap year.
Others tipped for the honour include Kate's sister Pippa Middleton and Prince Harry.
The new parents may also have asked Kate's Marlborough friend Alicia Fox-Pitt, Hugh van Cutsem, the son of Prince Charles' confidantes and James Meade, son of former Olympic equestrian Richard Meade.
The christening is due to take place in the autumn when the Queen is back from holiday in Balmoral. George will wear a replica of the Honiton lace gown used by Queen Victoria's children. It was made a few years ago when courtiers decided the original was too fragile to continue in use at royal christenings.