The pop star had Geoffrey James on the piano as she sang
Singer Jessie J surprised passers-by as she performed at St Pancras International station on Tuesday.
The 37-year-old, whose real name is Jessica Cornish, was accompanied by Geoffrey James on the piano.
Crowds watched on as she sang, some smiling and recording the performance on their phones.
It comes after the mother-of-one announced in June that she had been diagnosed with early breast cancer, with her first surgery taking place some weeks later.
In August, she revealed she had to undergo a second round of surgery before the end of the year, and postponed her upcoming tour.
Last month, Jessie J shared a hug with the Princess of Wales after the Royal Variety Performance, which the singer said was prompted by their shared experience of cancer.
After her conversation with the princess, who wore a floor-length green, velvet gown, the two embraced.
The singer-songwriter has had three number one songs in the UK singles chart with Domino, Price Tag and Bang Bang.
She was awarded four Mobo awards in 2011 including best UK act, best newcomer, best song for Do It Like A Dude and best album with Who You Are – and won the Brit Award for rising star in 2011.
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