William and Kate's son was given a mini rubgy Wales shirt by the team's captain Sam Warburton when the Duke dropped by the locker room before the Wales vs South Africa final of the Prince William Rugby Cup.
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It was emblazoned with the lucky number seven and "George" – much to the delight of William, who has been vice patron of the Welsh Rugby union since 2007.
The new dad said he was "proud" to hold the position "during what has been a truly memorable time" for rugby.
He continued: "And I am sure that – given the strength of the squad – this is only the beginning of a great period for Welsh rugby."
During a tour of the Millenium stadium William was taken to admire a portrait of his grandmother done by Cardiff-born artist Dan Llywelyn Hall and met former players who make up the up the Welsh Rugby Charitable Trust.
He then took his seat in the stands to watch Wales take on the South African Springboks.
Earlier in the week during a visit to St Giles Trust in London, the Duke of Cambridge said that he would have his "fingers crossed" for Wales, and predicted a "brutal match".
Despite his best efforts of cheering his side to victory, Wales lost out 15-24 to South Africa.