The prospect of Prince Harry marrying girlfriend Meghan Markle has been discussed with the Duke of Cambridge - who responded with a diplomatic laugh.
Harry and Meghan's romance appears to be hitting new heights after they held hands and gazed lovingly at each other during an appearance at the Invictus Games in Toronto.
Their public show of affection has even prompted bookies to offer odds on the couple getting engaged before the end of the Games this weekend.
When William visited Milton Keynes to celebrate its 50th anniversary he met craftsmen, based close to the new town, who make the vellum for royal documents and for use in parliament.
Paul Wright, general manager for William Cowley Parchment Makers, told the Duke: "If Prince Harry marries Meghan then his certificate will be in our vellum."
Under the Royal Marriages Act 1772, all descendants of George II must obtain the sovereign's agreement before they wed, otherwise the marriage is invalid.
The Instrument of Consent is an elaborate document made from vellum, usually calf skin, proclaiming the monarch's approval of a marriage and bearing a large red wax Great Seal of the Realm.
King George III, George II's grandson, ordered the act after his younger brother the Duke of Cumberland secretly married Lady Anne Horton, deemed to be a highly disreputable widow of a commoner.
William burst out laughing when told about the use the vellum could be put to and Mr Wright said about the Duke's response: "He just laughed - a nice laugh."
Bookmakers William Hill are offering odds of 16-1 that Harry and Meghan are engaged before the Games end, 6-4 odds if the announcement is later in the year, and 1-3 for the news to be released in 2018.
William Hill spokesman Rupert Adams said: "They look very happy in each other's company and I for one expect an announcement imminently."
William arrived in Milton Keynes by train, believed to have been a scheduled service, and was taken to Campbell Park to tour a small festival showcasing the best of the new town from technology and innovation to charities and the arts.
Before chatting to the craftsmen who made the vellum he met a group benefiting from a mental health service run by the Sports and Education Trust of the local football club MK Dons, which stages seven-a-side football matches for those with psychological problems.
When William caught sight of the group around a table football game, he said "fantastic", then spotted one team had the strip of his club, Championship side Aston Villa, stuck on the players, while the other was dressed in the colours of the MK Dons.
He grasped the handles controlling Villa's midfielders and attackers and smiled as he stared down at the players.
As the game began Luke Smith, 18, from Milton Keynes, joked with the future king telling him "come on mate" as the ball bounced around.
At one point the Duke was reminded he had to move on to keep to the schedule but said "after we've scored one more," and his team eventually won 2-0 - but both strikes were own goals.
The 18-year-old said later: "He noticed the shirts we put on the players and he's quite good - I scored both of his goals."
Speaking about the benefits of playing real football, Mr Smith added: "You get a good high from chemicals running through your brain and playing in a team you get a good sense of unity."