Elton John and David Furnish
Furnish, who is married to Sir Elton John, is Canadian but has been living in the UK for 27 years and has now become a dual passport holder for both countries.
The 54-year-old posted a photo to his Instagram page showing himself and the couple's young sons Zachary and Elijah holding hands as they face a portrait of the Queen and a Union Flag.
He captioned the photo: "A very emotional morning for me. This morning, I was sworn in as a British citizen.
"As an immigrant, I moved to the UK more than 27 years ago. I was welcomed with open arms.
"This spirit of acceptance, tolerance and diversity has allowed me to achieve a life beyond my wildest dreams.
The 54-year-old posted a photo to his Instagram page
Elton John's Oscars after party
Mon, February 29, 2016
Stars arrives at the Elton John AIDS Foundation Academy Awards party in West Hollywood.
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Emily Ratajkowski, Heidi Klum & Mariah Carey attend Elton John's Oscars after party
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"I am a very blessed man, and I am extremely proud to now carry both British and Canadian passports. GBCA @eltonjohn #ShareTheLove."
Furnish and Sir Elton began their relationship in 1993 and had a civil partnership on December 21, 2005, the first day that the ceremony could be performed in England, then converted it into a marriage in 2014.