John, 67, and Furnish, 52, planned a celebration at their estate near Windsor Castle, where an A-list crowd was expected, the Associated Press reports.
The singer's rep told the AP earlier this week that the ceremony would be private.
The couple, who have been together since 1993, are already parents to Zachary Jackson Levon Furnish-John, who will be 4 years old this Christmas, and Elijah Joseph Daniel Furnish-John, 23 months.
A law passed earlier this year in England allowing same-sex marriage and the Grammy winner has always been open about his relationship with Furnish, telling Parade magazine in 2010 that he "was attracted to David immediately."
He regaled the magazine on how they met.
“In 1993 I went back to my house in Windsor for a while. I wanted meet new people so I rang up a friend in London and said, ‘Could you please rattle some new people together for dinner here Saturday?’” John told Parade. "[David] was very well dressed, very shy. The next night we had dinner. After it, we consummated our relationship. We fell in love very quickly.”
John added that to keep that spark in their relationship, "Every Saturday for 16 years, we’ve sent each other a card, no matter where we are in the world, to say how much we love each other.”
The couple famously already entered into a civil partnership in 2005.