Fifa has climbed down over its decision to ban the England football team from wearing remembrance poppies and will now allow players to display them.
Pressure from the Duke of Cambridge and the Prime Minister helped persuade football's international governing body to reverse its decision.
William penned a strongly worded note privately to Fifa expressing his dismay at the move.
David Cameron also wrote to the organisation, assuring it that there were "no political connotations whatsoever" associated with the poppies.
Less than an hour after the letters were made public, Fifa announced that it would allow England's players to wear a poppy on their black armbands during Saturday's friendly match with Spain.
A spokesman for St James's Palace said William, who is president of the Football Association, was pleased with the decision, he said: "The Duke is happy with this resolution."
A Downing Street spokesman said Mr Cameron welcomed Fifa's decision, saying: "It's a sensible way forward."
A Royal British Legion spokesman said: "The Royal British Legion is delighted that England players will be able to wear their poppy with pride.
"As we said earlier today, there are other ways of honouring the poppy than wearing it on your shirt - the armband will do nicely. Now everyone can concentrate on enjoying the game."
