The ceremony will take place in the Windsor Guildhall, the same venue as the civil wedding of Britain's Prince Charles and Camilla.
Elton John’s announcement came on the day that the new laws came into force in Britain allowing gay couples to become legal partners with the same legal rights as heterosexuals, and follows mounting speculation about a venue.
Civil partnerships or "gay marriage", confers legal status on homosexual unions, allowing similar tax and inheritance benefits as heterosexual couples.
Same-sex marriage in church, however, is still banned, while homosexual unions themselves have been the subject of controversy in the Church of England, whose worldwide Anglican communion is already divided over gay priests.
The first civil partnership ceremony is scheduled to take place on December 19 in Northern Ireland, followed by Scotland on December 20 and England and Wales the following day.
Couples have to first register their intent, wait a compulsory 15 days and then go to a register office or other authorised place.
There they sign an official civil partnership document in front of a registrar and two witnesses. The ceremony is much like a civil wedding.
Royal wedding
Now pop royalty will not only follow the heir to the British throne by legally binding his relationship with the Canadian filmmaker in Windsor, west of London, but have the same official to oversee events.
Clair Williams, or the Superintendent Registrar of the Royal Borough of Windsor to give her full title, will officiate at the private December 21 ceremony, alongside experienced fellow registrar Peter Edwards.
Ms Williams, who performed the royal civil wedding at the Guildhall in April this year, said she was "delighted" the celebrity couple had chosen the historic 17th century venue.
"I and my colleague, Peter, will do out utmost to ensure that the ceremony is a very special and private occasion for them and their guests," she added.
Local authorities in Windsor, famous for its cobbled streets and fortified royal residence, are already liaising with police in anticipation of fans hoping to catch a glimpse of the couple, who have been together for 12 years.
The marriage to Furnish will be John's second.
The singer had wed German music engineer Renate Blauel in 1984 in Sydney, Australia, but they divorced four years later.