An organist whose duet with a passing London railway security guard went viral has said it all started with the two of them singing the national anthem.

Anna Lapwood, the director of music at Pembroke College, Cambridge had stopped to play the organ at London Bridge Station on Sunday.

She was approached by Marcella, a security guard who revealed she was also a classically trained singer.

The pair originally performed the national anthem, God Save The King, and then at the request of Marcella, Ms Lapwood launched into a rendition of Lascia ch’io pianga by Handel.

Ms Lapwood, 27, told the PA news agency: “I just thought that providing the musical accompaniment to people’s grief, even for five minutes, might be a positive thing.

“It was lovely because I think I played for about three hours in total in the end, which I was not planning at all.

“Sometimes it was just me and sometimes it was a really big crowd, like when Marcella was singing.

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“And everyone joined in with the national anthem at one point and it felt like a lovely way to help people and give them what they needed at that point in time- even if they didn’t realise they needed it.”

After Ms Lapwood posted the clip on Sunday it managed to gain three million views on Twitter.

Ms Lapwood added: “Marcella told me that she’d had some training in the past and used to be a church organist as well.

“She asked if I knew this song by Handel and by complete coincidence I had it on my iPad from a duet I had done with someone else last week.

“It was then that you really heard her voice open up and come to life.

“People started clapping and it was just so beautiful.”

Ms Lapwood is now attempting to find Marcella in the hope that the success of the video will encourage her to perform again.