The Gossip Girl lovers were staying at the Hard Rock Hotel in Chicago at the weekend (07-08Aug10), where they were reportedly lined up for a public appearance.
Organisers claim they paid for Westwick and Szohr's flights and VIP tickets to the Lollapalooza festival on the condition they appear at the Hard Rock event, but they reportedly stayed in their rooms and racked up an $800 (£533) bill instead.
A source tells New York Post gossip column PageSix, "Jessica was invited and took Ed as her plus-one. They were flown first-class and given two rooms, and her talent wrangler, Aaron, agreed they would appear at night or at one of the day lounges. But they never showed up, and instead they hit the minibar and ordered room service. The bill for food and booze for three nights was over $830.
"Publicists were calling their room and banging on their door. Eventually this blew up into a big argument on Sunday... They left that night, and Jessica paid for everything."
But reps for both stars have slammed the allegations, insisting the pair would have turned up to the event if they had known they were expected.
A spokesperson for Szohr says, "If Jessica had known, she would have gone. She was never told until Sunday, and as a result she agreed to pay for everything herself and left. There was no fight."
Westwick's rep adds, "Ed went as Jessica's plus-one and had no idea about any agreement or photographs he had to take. He always honors any agreements."