The Bangor based band were forced to cancel their first festival headlining gig as lead singer Alex Trimble has taken ill.
In a statement on Wednesday, the band said: "It is with a heavy heart that we are going to need to cancel our upcoming shows at Latitude and Bournemouth Academy.
"Alex collapsed at Seattle airport on the way to London and has been hospitalised with a chronic stomach complaint.
"We are devastated to have to cancel shows that we were so looking forward to. We can only apologise and hope for your understanding at this time."
After Allen was asked to step in for the band, she wrote on her Twitter page: "Sadly @TDCinemaClub have had to pull out of their headline slot at Lattitude festival, I am filling in for them, I hope that's ok."
The singer was met with online abuse following the news, prompting her to write: "If you're going to be rude about my replacing @TDCinemaClub on here, can you just un (follow) me please? I'm so exhausted by the nastiness.
"I understand that you're disappointed but I don't think that warrants abuse. They asked, I accepted, I will give it my all, and hope you enjoy."
Two Door Cinema Club gave Allen their backing by posting on their Twitter page: "You have our unequivocal support. You're a greatly talented artist! We know you'll absolutely smash it."
On Thursday, the band also announced they would be unable to perform at Australia's Splendour in the Grass festival as a result of Trimble's illness.