Stevie Nicks says she didn't even know Fleetwood Mac singer Christine McVie was ill until Saturday in heartbreaking farewell letter to her 'best friend'

• Stevie Nicks penned a touching tribute to "best friend" Christine McVie on Wednesday.
• McVie's family announced that the singer-songwriter died "peacefully" at the age of 79 this week.

Stevie Nicks honored her "best friend" and bandmate Christine McVie in a touching tribute on Wednesday, hours after McVie's family announced the singer-songwriter's death at 79.

The two members of Fleetwood Mac spent four decades making music and touring together, along with bandmates Mick Fleetwood, John McVie, and Lindsey Buckingham.

"A few hours ago I was told that my best friend in the whole world since the first day of 1975 had passed away," Nicks wrote in a handwritten letter posted to Twitter. "I didn't even know she was ill until late Saturday night."

McVie died "peacefully" in the hospital on Wednesday following a "short illness," a statement from her family said. She was surrounded by family members when she passed, the statement added.

Nicks said she wanted to travel to London to visit the sick songwriter as soon as she learned, but said "we were told to wait."

"So, since Saturday, one song has been swirling around in my head over and over and over," Nicks wrote. "I thought I might possibly get to sing it for her, and so, I'm singing it to her now."

On a separate piece of paper, Nicks copied lyrics from the band Haim's "Hallelujah," a song written about the death of member Alana Haim's best friend.

"I always knew I would need these words one day," Nicks wrote. "It's all I can do now."

Nicks ended the tribute by saying she would see McVie "on the other side, my love."

"Don't forget me," Nicks added.