The X Factor boyband were due to appear at the shopping centre earlier today, but security halted the event because too many fans had turned up.
An X Factor spokesperson said: "Union J's meet and greet had to be cancelled as the huge number of people there meant it wasn't safe for them to appear."
A source added: "The boys are gutted they couldn't meet their fans as they were overwhelmed on hearing how many people had turned up.
"But with so many fans there they were in danger of being mobbed and fans could have been crushed in the crowds."
The group later posted a photo on Twitter to "show how sorry [they] are", insisting that they feel "awful" for letting fans down.
"We are SO sorry, just about to leave BUT the security have told us we r not allowed 2 come as its a security risk. We feel awful. So sorry," they tweeted.
"To all u amazing people who turned up we are so overwhelmed for your support..it means the world. Tweet us a pic of u now and we will follow."
One Direction, who appeared on The X Factor in 2010, recently revealed that they are fans of both Union J and District3.
Photo gallery - X Factor week two:
Copyright: Ken McKay/Thames/Rex Features