Phoebe Waller-Bridge’s sister Isobel has released a new single, which she describes as “intimate”.

The classical composer came into the public eye after writing the music for hit comedy Fleabag, which was written by and starred her sister.

Isobel has also written the scores for other films and shows including Emma, Black Mirror and Vanity Fair.

Phoebe has previously spoken about people assuming Fleabag’s uptight older sister in the show – Claire – is based on Isobel, and her need to defend her family against all the attention following the comedy’s success.

Describing her new single September as her “most personal project”, Isobel said it would be a “hello” to her new record label Mercury KX.

The track won’t be on her second album, which the 35-year-old is currently recording, but will appear on a compilation album called Flow.

Isobel, who began playing the piano aged four, is now working in her own recording studio in Hackney, east London.

She said: “This has been my dream since my teens. I still can’t quite believe it’s happening.”

The single was inspired by an improvisation, and Isobel says it reflects “the very personal journey I am about to go on”.

She has been reunited with her baby grand piano after eight years in storage, and says she is now ready to get back to work.

Describing her new album as “unique”, she hinted that she might work with a cinematographer to “create a whole new world” with her music.

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Isobel compared it to “going through a door and, by the end of it, you can see the mountain”.

She released her first album – Music For Strings – in 2015.

In keeping with their creative talents, Phoebe and Isobel also have a younger brother, Jasper, who is a music manager.