Taylor Swift.

Taylor Swift made a surprise appearance at Haim’s sold-out concert in London’s O2 Arena on Thursday (21 July).

The American rock band comprising sisters Alana, Este and Danielle Haim were midway through performing their hit “Gasoline” when they stopped suddenly.

Introducing Swift, 32, Alana said: “Wait, you guys. We’re in London, we can’t play London without bringing out a special guest.

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“Because not only is this person that I’m about to bring out the most incredible singer, songwriter, producer, f***** amazing person all around, she’s one of our close friends,” she continued, as Swift walked onstage to thunderous applause and cheers.

The “Bad Blood” singer noted she hadn’t “been on stage in a very long time” but that “it’s very nice [to be back]”.

She continued: “When I heard my girls were playing in London at the O2, I thought, I’m going to have to see that. And it looks like there’s about 20,000 other people who also thought that.

“So we had a thought. If we were to do some sort of mash-up, we could possibly maybe get you to sing the loudest you have sung all night,” Swift told concert attendees, adding, “Which is a big challenge because you have been singing very loudly – extraordinary work.”

Then, as promised, Swift and Haim delivered a mash-up of “Gasoline” and Swift’s 2016 solo hit “Love Story” amid a frenzy of fan TikToks and thunderous applause.

Fan videos of the performance were posted on social media as those in the attendance at the O2 were left “screaming” over Swift’s surprise cameo.

Along with a video of the “Gasoline”/”Love Story” mash-up, one person wrote: “HAIM BROUGHT OUT TAYLOR SWIFT!!! I’M SCREAMING.”

Another fan tweeted: “Taylor Swift just walked out on stage at the O2 with Haim. Girlfriend is in meltdown”.

“No one in the Taylor Swift universe is ever going to know peace now that she went and spontaneously performed with Haim – like she really could show up any place any time,” one person commented on Twitter.

Haim opened for Swift during her 1989 tour in 2015. The group also featured on Swift’s 2020 album evermore, collaborating with the singer on the track “No Body, No Crime”.