Scarlett Johansson has admitted that she felt “embarrassed” singing in Ray Gunn.

The American actress, 41, plays the character of Venus Nova, a famous pop star struggling from a scandal that threatens to destroy her career in the animated film, which premiered at the BFI’s London Film Festival on Saturday.

Speaking to The Press Association on the red carpet, Johansson said singing in the film was the “hardest part”.

“As far as the songs go, I find that part to be the most frustrating because it’s not my everyday job. I love to sing and I sing all the time, but doing it in the recording studio with music producers and the cameras – that part felt daunting,” she said.

“I lose my nerve in those moments sometimes. But you just get through it and keep trying and eventually after you go through the process of feeling kind of embarrassed and resigned, all that melts away and you are able to do it.”

The actress is best known for starring in films including Lost In Translation, Black Widow and Marriage Story, but has sung in several movies and soundtracks throughout her career, including as Ash the punk-rock porcupine in the animated musical franchise Sing and Sing 2.

Directed by Brad Bird – who also co-wrote the film – Ray Gunn is set in an alternate 1939, in the giant futuristic city of Metropia – a corrupt world where humans and aliens co-exist, but the latter are the dominant species.

Sam Rockwell stars as the titular character – a downtrodden private eye who investigates a murder, but the film’s synopsis has teased that “all is not what it seems”.

Also speaking to PA at the London premiere, Bird said Rockwell and Johansson were “magnificent actors who convince you they are feeling and doing something for the first time when they do it, and that’s a rare gift”.

“In Scarlett’s case she is not only an amazing actress but she can really sing too, and she sings in this film too and that’s rare,” he added.

Rockwell added that his role included “a lot of grunting and screaming”.

Bird is best known for writing and directing Pixar’s The Incredibles and Ratatouille, which earned him two Academy Awards for best animated feature and best original screenplay nominations.

Ray Gunn also stars Tom Waits, Matt Berry, Bobby Cannavale, Michelle Rodriguez, Matt Jones, Aasif Mandvi, Alan Tudyk, and Kathryn Hunter.

It is set to be released in cinemas on December 4, and on Netflix on December 18.