Elton John marries longtime partner David Furnish

Legendary singer Elton John and his partner David Furnish have formally converted their civil partnership to a marriage here in Windsor estate.

"That's the legal bit done. Now on to the ceremony," John, 67, posted on his Instagram page along with a photo of himself, pen in hand, sitting next to Furnish at a desk with a piece of paper in front of them.

The couple will host a ceremony at their Windsor estate in Berkshire later and a number of stars are expected to attend, a spokesman said, reported BBC online.

John and Furnish tied the knot in a civil partnership in 2005. In March, the law was changed in England and Wales to allow same-sex marriage.

The 'Sacrifice' hitmaker took to Twitter last evening to direct people to the couple's instagram accounts, tweeting, "Join Elton John and David Furnish as we share special moments from our Wedding tomorrow. ShareTheLove."

The singer has already posted snaps on the photo-sharing website, including an image of a wedding invitation emblazoned with red roses which confirms the big day is on December 21.

The couple are already parents to two sons -- Zachary and Elijah -- both born through surrogacy.