One Direction called an ambulance to a fainting "fan" - then found out they'd been pranked by Ant and Dec.
The 'Little Things' hitmakers "panicked" after a man they were told was their biggest fan in New Zealand hyperventilated and suffered an asthma attack, so called in emergency services, before the 'Saturday Night Takeaway' hosts revealed their true identities.
Ant revealed: "The ruse was that 1D's biggest fans had flown in to meet them.
"Dec dressed up as one called Jack from New Zealand and I was Ben, a TV host from there.
"I introduced him to the group - and the meeting didn't go as planned. But they fell for it."
Dec added: "I faked an asthma attack. They panicked and I hyperventilated.
"They called medical emergencies. It was such fun and they fell for it.
"We did it in Madison Square Garden before Christmas."
The gag also saw Ant "accidentally" leave his microphone switched on, leaving the group squirming as he talked about who was the sexiest member.
And Dec revealed the prank had been well thought out for some time.
He told the Daily Mirror newspaper: "On the 1D tour they met two fans from every nation.
"We heard the only two who didn't go were from New Zealand and thought, 'This is our way in'. We were all over them."
The prank will be broadcast on the show on Saturday.