The singer, who scored a top three hit in the U.S. last year (14) with Bang Bang, cancelled a number of dates on her Australian tour earlier this month (Mar15) after she was struck down with a mystery illness.

Jessie J was bedridden for several days, but she flew back to her native Britain to keep a commitment with bosses of MCDonald's to promote the fast food chain's new imlovinit24 campaign with a special gig in London on Tuesday (24Mar15).

The pop star performed an intimate show on an open top bus for a group of competition winners, belting out her hits as the vehicle drove around central London.

She says of her partnership with MCDonald's, "I'm delighted to be part of the MCDonald's imlovinit24 initiative helping to spread some happiness over the next 24 hours in so many countries. Travelling around London... with my fans... (is) brilliant, I love surprising them! Through imlovinit24.com fans all around the world can continue the fun."

Fellow singer Ne-Yo also joined the drive by playing a free gig at the Avalon Hollywood venue in Los Angeles on Tuesday night (24Mar15).