The "Sign of the Times" hitmaker performed a surprise set at Camden's Electric Ballroom on Thursday night, December 19, and brought Stormzy out as a guest to team up on the grime star's track "Vossi Bop".
Harry joined fans in singing the song's anti-establishment line, "F**k the government and f**k Boris," a reference to newly re-elected British Prime Minister Boris Johnson, who won the U.K. general election last week, despite Stormzy backing his opponent Jeremy Corbyn.
Footage posted on Twitter also showed the "Shut Up" hitmaker praising the One Direction star, despite both their new records competing for the Christmas number one spot in the U.K. albums chart.
Stormzy said on stage, "They tried to make it a chart battle. This isn't a chart battle, this is my f**king brother. I wanna say this on record... this guy has made a f**king brilliant record."
He then added, "From my heart bro, you're not just a pop star, you're a brilliant artist."
The 25-year-old also performed a version of Lizzo's "Juice", a track he covered in a BBC Radio 1 Live Lounge session on Wednesday.
Fans will find out whether Harry's "Fine Line" or Stormzy's "Heavy" is the Head will top the yuletide chart on Friday, with Rod Stewart's "You're in My Heart" also vying for the crown.