The couple were one of the first to form a civil partnership on 21 December 2005 but want to take their relationship one step further nine years later, now that same-sex marriage is legal in England.
“We had our big shebang when we had our civil union but we will (have a wedding),” John told Sky News earlier this year.
“I think it’s a great opportunity to take advantage of the new laws and to celebrate. We’re very lucky in this country to be able to have that advantage and to have that position to be in.”
Elizabeth Hurley, the Osbournes and the Beckhams are among the star guests expected to attend the ceremony, while John and Furnish’s two sons, Elijah and Zachary, will be acting as ring bearers.
John's spokesman Gary Farrow said on Wednesday that the wedding will be a private affair.
Britain is the 15th country to legalise same-sex marriage, which is also legal in several US states.
The new law allows gay couples to convert their civil partnerships to marriages. The day it became legal - 29 March - became a landmark moment for a country that little more than a decade ago had a law banning the “promotion” of homosexuality.