Charli XCX was 'honored' to write song for Britney Spears: 'What a queen'

March 4 (UPI) -- Charli XCX says she was "honored" to write a song for fellow singer Britney Spears.

The 29-year-old British singer discussed writing a song for Spears' album Britney Jean during Thursday's episode of Watch What Happens Live.

Charli XCX wrote and submitted a song for Britney Jean that ultimately didn't end up on the album. Charli XCX said working with Spears was still an "amazing" experience.

"Working on the album was, you know, amazing. Nothing ended up on there but I'm honored anytime Britney calls. What a queen," she said.

Charli XCX also addressed rumors that she was Spears' first choice to appear on the "Slumber Party" remix.

"I don't know if I was the first choice but I did get asked to feature on the 'Slumber Party' remix," she confirmed. "Unfortunately, it just couldn't work out with the timings and everything."

Charli XCX said she's open to working with Spears again in the future.

"I would love that," she said.

Spears released Britney Jean in 2013. The album features the singles "Work Bitch," "Perfume," "Til It's Gone" and "It Should Be Easy."

Charli XCX has written or co-written several hit singles for other artists, including "Same Old Love" for Selena Gomez, "Tonight" for Blondie and "Señorita" for Shawn Mendes and Camila Cabello.

As a singer, Charli XCX is known for the singles "Boom Clap," "Break the Rules" and "Beg for You" featuring Rina Sawayama. She will release her fifth studio album, Crash, on March 18.