They are brothers but this morning Princes William and Harry will also be bitter rivals.
Prince Harry is an avid England fan and honorary president of England Rugby, but his older brother, the Duke of Cambridge, is vice-patron of the Welsh Rugby Union.
The brothers were expected to sit sideby side in the stands at Twickenham for the England-Wales match at 8am today - but will each wear a replica shirt of their team.
Yesterday, Prince Harry called on fans to create an "incredible atmosphere" to help spur England to victory and give him bragging rights over his brother.
"It doesn't get any bigger for the players than this - pulling on their national shirt and representing their country at a World Cup," he said.
"So please join me in showing them our support and help create an incredible atmosphere up and down the country."
The Duke has issued a similar plea to the Welsh team.
He told the players at their World Cup welcome ceremony in London: "Unfortunately, I will be watching you with my brother, so I will need a Wales win more than ever."
This morning's match is the biggest of the World Cup so far. Neither side can afford to lose as only two teams qualify from Pool A, which also includes Australia.
England hoped to make the most of the home advantage in the 82,000-capacity stadium, but as many as 30,000 Welsh fans are believed to have tickets. The royal household is not the only family with divided loyalties.
Cousins Billy Vunipola and Taulupe Faletau grew up together playing rugby in their back gardens in Pontypool, South Wales, after their fathers, who played for Tonga, moved there in 2000.
The Vunipolas moved to Bristol, so this morning Billy, 22, plays for England and his cousin is in the Wales team. Vunipola's brother Mako, 24, is among England's substitutes.
