Justin Timberlake covers Grease ballad

The pop star was launching his second album this year at the show

Justin Timberlake has surprised fans by singing Beauty School Dropout, from the musical Grease, during a gig in London.

The singer was explaining he liked the UK because it was "the one place away from home where you can kind of still understand what I'm saying".

"I can barely speak English," he added. "I'm a beauty school dropout."

The 32-year-old then launched into an impromptu a capella rendition of the ballad, sung by Frankie Avalon in the 1978 movie adaptation of Grease.

"I just dated myself," joked the star. "Half of the crowd is like, 'I don't understand!'"

The singer's 60-minute set was streamed to more than 100 countries, from Australia to Kazakhstan, as part of the iTunes Festival.

Adele, Benedict Cumberbatch and Downton Abbey star Elizabeth McGovern were among the celebrities who watched the show live at Camden's Roundhouse.

Timberlake, sharply dressed in a Mad Men-style suit, previewed several songs from his new album The 20/20 Experience: 2 Of 2, released on Monday.

Backed by 13 musicians, including a four-strong brass section, Timberlake eschewed his troupe of backing dancers and concentrated on the music.

He strapped on an acoustic guitar for the hit single Like I Love You and played an electric piano during the Flamenco-influenced Senorita.

But he still managed to break out a few Jackson-esque dance steps during the exuberant disco funk workout, Take Back The Night.

As well as Beauty School Dropout, Timberlake incorporated elements of Led Zeppelin's Kashmir into the ballad Cry Me A River, and referenced The Doors' Break On Through To The Other Side during set-opener Only When I Walk Away.

The singer-turned-actor has just announced a string of UK shows, for March and April 2014.

The month-long iTunes Festival, which has featured the likes of Lady Gaga, Elton John, Avicii and Rizzle Kicks, ends on Monday with Katy Perry's first UK show since 2011.