The ceremony is expected to take place in Windsor, outside London.
John told England's Daily Mirror newspaper that he's very excited to marry.
"We definitely want to do it about the middle of December," John, 58, told the paper. "Meeting David has been the greatest thing to happen to me." Gay civil partnerships become legal in the U.K. on December 5.
John's publicist Gary Farrow says there's a strong financial incentive for the two to formalize their relationship
On December 5th, British tax laws will give gay civil partners the same breaks as married couples.
There's been a lot of contradictory speculation about the pair recently, with Furnish denying one report that they were about to split up after more than 11 years together, reports CBS News Correspondent Steve Holt.
John has credited 42-year-old Furnish with helping him to overcome his life-threatening drug and alcohol addictions.
The pop icon was previously married to studio technician Renate Blauel; the pair wed on in 1984 and divorced in 1988.
Windsor was the scene for Prince Charles and Camilla Parker Bowles' wedding earlier this month.
John was a friend of Diana, and his 1997 tribute to her, a re-recording of "Candle in the Wind," passed Bing Crosby's "White Christmas" to become the best-selling song of all time.
However, the singer told the Daily Mirror he supports the prince's marriage to Parker Bowles marriage, saying, "Let them be happy. We live in the 21st century, for God's sake."
"People tell me I shouldn't really be for it because I was a huge friend of Diana, but her marriage to Charles was a bit like my parents'. Maybe they shouldn't have got married in the first place, because they weren't suited," he says.