A clip posted by the star on Twitter shows 450 actors playing MPs singing a rousing rendition of The Beatles' Hey Jude in the House of Commons.
They then applaud as Gary Oldman, who plays Churchill, enters the room.
West, who plays former foreign secretary Sir Anthony Eden, wrote alongside the video: "Been waiting to post this for a year. Joe Wright, director of Darkest Hour, leads 450 MPs in a warmup sing of Hey Jude between takes. Gary Oldman as Churchill makes an entrance at the end."
West also shared a series of photos to celebrate the film's release, including snaps of Oldman's Churchill at the despatch box and director Joe Wright.
The political chamber was recreated at Leavesden Studios near Watford. Kristen Scott Thomas, who also stars in the film, told Sky News it was a "very magical set".
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The film also stars Lily James, Stephen Dilliane and David Schofield and focuses on Churchill's handling of the Second World War.
James has spoken of the cast's "audible gasp" at seeing Oldman's transformation into Churchill.
Source : https://news.sky.com/story/darkest-hour-mps-sing-hey-jude-in-recreated-house-of-commons-11205484437