The classical duo will round off an impressive month of music in June at the royal landmark in London, which sees the likes of Nile Rodgers & Chic, Kylie Minogue, The Jacksons and Tears For Fears perform, on June 22.
The pair - who were named Best Group at the 2018 Classic BRIT Awards, and have had two number one albums to their name - last played the idyllic Palace Gardens in 2017.
The final headliner announcement comes after Nile Rodgers and his band added a second show at the extravaganza due to demand.
The 'Le Freak' hitmakers - who released their first album in 25 years, 'It's About Time', in September - sold out their first date on June 14, and will now play a second concert at the 3,000 capacity Base Court - one of the Tudor courtyards in the stunning grounds of the historical landmark - on June 15.
Caro Emerald will play the festival on June 12, jazz star Jools Holland performs on June 6, new-wave pioneers Tears For Fears on June 18 and 19, and pop legends The Jacksons on June 7.
Kylie will headline on June 20 and 21, for what will be the Australian pop princess' only London dates of the summer.
The finale night will also see a fireworks spectacular over the palace.
Tickets for June 22 go on general sale at 10am on Thursday (14.03.19).
VIP packages are available by visiting www.hamptoncourtpalacefestival.com
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