The Duke of Cambridge kicked off the first football match to be played at Buckingham Palace - after warning that any players breaking windows would answer to the Queen.
Two of England's oldest amateur clubs - Civil Service FC and Polytechnic FC - took to the pitch marked out in the Queen's garden for a competitive amateur league fixture.
Before kick-off, William, who helped to organise the match, shook hands with the players and met referee Howard Webb, one of the country's best-known football officials.
He has performed at the highest level, having taken charge of World Cup and Champions League finals and the recent Premier League Manchester derby match.
The Duke was joined by former England striker Michael Owen and FA chairman Greg Dyke and joked with the Polytechnic players: "Michael's available as a super sub."
The two teams, who play in the senior division one of the Southern Amateur League, warmed up in the autumn sunshine before the match began.
Civil Service FC is the sole surviving club out of the 11 which founded the FA in the Freemasons' Tavern in Great Queen Street, London, in 1863 and later drafted the 13 original laws of association football. Polytechnic FC was formed in 1875.
The Civil Service team presented William with two tiny tops for his son, Prince George - one red and the other white, both with "HRH 1" on the back.
Earlier, the Duke paid tribute to football's unsung heroes by honouring 150 grassroots volunteers during a Palace reception.
William, president of the Football Association, presented the hard-working helpers with medals recognising their efforts, part of the FA's 150th anniversary celebrations.
He told the guests: "At its best, football is a powerful force for good in society. It binds people from different backgrounds, communities, faiths and abilities - and gives them a common interest, a unifying identity.
"I believe, over its 150 years, football has remained a wonderful example of the power of community and of our ability to come together to organise and to enjoy a simple pastime."
William joked that if a window was smashed, the footballer responsible would have to face his grandmother.
He said: "This magnificent home, Buckingham Palace, is at the heart of the nation, and so there cannot be a more fitting setting to celebrate our national game, and to celebrate all of you."
Speaking about the Queen, he added: "One warning, though: if anyone breaks a window, you can answer to her."