"It's an incredibly brave thing to do, especially when you're older," said the singer, "I admire him for doing it and I support him 100 percent."
"People should support him," added Furnish.
Another British singer also voiced his support for the former Olympian . Boy George told People, "I applaud what he's doing. I think it's very brave. And it actually makes me quite emotional," he said earlier this month.
Jenner is expected to address the transition, which has been the topic of tabloid speculation for months, in the interview with Sawyer.