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The second heat's six groups include an all-female pop choir called The City Academy Songbirds, a group of soul singers known as Soul Train, a group of five childhood friends from Loch Lomond called Scot Soul, YouTube a cappella sensations Out of the Blue, rat pack-inspired quartet The Flyboys and pop choir The Noteables.

To progress, the groups must impress the panel of musical judges - the nation's favourite choirmaster Gareth Malone, former choirgirl turned international recording artist Kelis and superstar guest judge international soul sensation Chaka Khan.

The programme opens with an epic musical mash-up, arranged by the series musical director Deke Sharon, who is the musical director on the Pitch Perfect movies.

Groups then go head to head in a series of knockout choral challenges. First they each perform their showstopper before battling it out with their opponent in a themed riff-off. The judges then decide which group goes through to the solo battles later on.

At the end of the programme only two groups will remain, where they must face the ultimate sing-off - the final battle. Here they share the same song, before superstar guest judge Chaka Khan joins in with the group the judges have chosen to go through to the final.

Chaka will return in the grand final to sing with the show's winning group, who will be one step closer to the 拢50,000 cash prize.