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 side by 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.