Olivia Rodrigo Recruits Lily Allen for Surprise 'Smile' Duet at London Show

Olivia Rodrigo gave fans another reason to "Smile" during her latest tour stop in London!

During her Guts World Tour stop at London's O2 Arena on Friday, May 17, the 21-year-old Grammy winner brought out Lily Allen for an onstage reunion — nearly two years after they last performed together at Glastonbury in 2022.

"The best day of my whole career was when I got to sing with her a few years ago at Glastonbury," Rodrigo told the crowd before introducing her fellow musician. "I absolutely adore her. Would you please give it up for Ms. Lily Allen!"

Once Allen, 39, joined Rodrigo onstage, the duo performed Allen's 2006 smash "Smile," taking turns singing lead and eventually doing some harmonizing for the chorus. They shared a lengthy hug at the end of the heartfelt reunion, as Rodrigo encouraged her fans again to "give it up" for Allen.

"Been a while since i’ve sang at the O2. what a lovely crowd," Allen reflected in an Instagram upload, which included a few behind-the-scenes photos from the surprise performance. "what a lovely crew. Thank you @oliviarodrigo x"

"Last night at the O2 wit the coolest girl in the whole world," Rodrigo wrote over a video of the performance on her Instagram Stories, before sharing a wholesome photo of the pair backstage.

Rodrigo and Allen previously teamed up at Glastonbury in 2022 for a performance of Allen's 2008 hit "F--- you," which Rodrigo at the time said was in dedication to "the five members of the Supreme Court who have showed us that at the end of the day, they truly don’t give a s--- about freedom," following the overturning of Roe v. Wade.

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories. 

While Rodrigo is busy touring her sophomore album Guts, her 2021 debut LP Sour just reached a major milestone, becoming the most streamed album by a female artist in Spotify history.

A press release from the company this week noted that Sour — which includes hits such as "Drivers License" and "Good 4 U" — "earned the most U.S. audio streams for a female debut album ever and broke the record for the most-streamed album in a week by a female artist on Spotify." A rep for Rodrigo also confirmed to PEOPLE that the debut album has earned more than 21 million consumption units worldwide.

Rodrigo also suffered something of an onstage wardrobe mishap this week, but took it in stride. While performing in London on Tuesday, May 14, the musician acted quickly when the back of her crop top unfastened, something she joked was "f---ing embarrassing" after she finished performing track "Love Is Embarrassing."

"I almost flashed you guys, but it's OK now," she said, laughing it off.

Rodrigo's Guts World Tour will continue through October with dates in the U.S., Asia, Australia and elsewhere.