Zayn Malik has blamed "anxiety" for his last-minute decision to pull out of the Capital FM Summertime Ball.
The music event at Wembley Stadium would have been Malik's first live UK solo stage performance.
In a message on his Instagram account on Saturday, the former One Direction star apologised "to everybody I've let down" and explained that his anxiety about performing live "has gotten the better of me".
He said: "To all those people who have been waiting to see me perform at the Capital Summertime Ball today.
"I flew into the UK last night to appear in my home country in front of my family, friends, and most importantly my UK fans.
"Unfortunately my anxiety that has haunted me throughout the last few months around live performances has gotten the better of me ... with the magnitude of the event, I have suffered the worst anxiety of my career."
The 23-year-old added: "I cannot apologise enough, but I want to be honest with everyone who has patiently waited to see me.
"I promise I will do my best to make this up to everybody I've let down today.
"I know those who suffer anxiety will understand and I hope those who don't can empathise with my situation."
Capital FM said in a statement: "We're very sorry we've just been told that due to illness Zayn will not be performing at today's show.
"We know everyone's already had a great day and enjoyed the 19 other huge artists' performances, but of course we're all gutted that Zayn's had to pull out tonight at the last minute."
The statement ended with Capital FM "wishing him better".
There were jeers from disappointed fans when Malik's absence was announced inside Wembley Stadium.
One person tweeted: "Never heard boos more deafening in my life than when they announced at 9:20pm that Zayn had pulled out of #CapitalSTB."
But fans flocked to support Malik and This Morning's Phillip Schofield tweeted that he had "huge respect for Zayn" for discussing his problem.
Malik's decision to pull out of the Summertime Ball follows a number of non-appearances from the Pillowtalk singer.
In January, the Bradford-born star pulled out of The Graham Norton Show due to "scheduling" issues.
A month later, he cancelled a Radio 1 interview with DJ Greg James, a move which was criticised at the time as "rude" by Good Morning Britain's Piers Morgan.
The Capital FM Summertime Ball, which raises money for charity, saw performances from Little Mix, Ariana Grande and Years & Years.