A source confirmed to ET on Wednesday that Grande was set to be a surprise guest at the awards show to pay tribute to the tragedy -- which took place outside of her May 2017 concert -- but had to pull out due to health reasons that kept her from flying.
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Ariana Grande Explains Why She Didn't Want to Call Off Her World Tour After Manchester Bombing
"Ariana fell ill and was going to come in spite of that, but was ordered by her doctor to not fly," the source said.
In Grande's absence, Gallagher took the stage with a touching performance of "Live Forever," with attendees like Ed Sheeran standing up and singing along to the song.
After the Manchester terror attack, Grande put together "One Love Manchester," a star-studded benefit concert that raised millions of dollars for the Red Cross' We Love Manchester emergency fund. For her efforts to help the victims' families after the tragedy, the 24-year-old singer was made an honorary citizen of Manchester last July.