A British man holding a nine-month-old baby was punched in the face by a member of staff at Nice Airport after remonstrating over a 14-hour delay, a witness has said.
Arabella Arkwright, 49, captured the blow in a picture circulated online.
An easyJet spokesman said the person in the photograph is believed to work for Nice Airport's special assistance provider Samsic.
"It was completely unprovoked," said Mrs Arkwright, who was returning from holiday with her husband, Johnnie, 64, their son Harry, 16, and daughter Violet, 13.
She told the Press Association: "This guy literally smirked, smiled as if, excuse my French, basically 'f*** you,' and then whack. It was a proper whacking.
"My husband pulled him off. You can see my husband's face, he's absolutely appalled. He's the most unconfrontational man in the world."
She said her fellow passengers "couldn't believe it", adding: "They were yelling, 'get the police, how dare you.'"
Mrs Arkwright, who runs tourist attraction Hatton Country World, in Warwickshire, said passengers' frustrations grew during the delay to their flight to Luton.
"These were people who had had a lovely family holiday, who will remember this and think, 'oh my God,' – children crying, no nappies, no formulas, nobody allowed out," she said.
"All the easyJet employees at the airport were perfectly nice, very young, perfectly nice, they were non-aggressive. It wasn't their fault. The structure does not work. Basically, they were cannon fodder."
A spokesman for the airline said: "EasyJet is very concerned to see this picture and can confirm the person in the photo is not an easyJet member of staff and they do not work for easyJet's ground handling agents in Nice.
"We are urgently taking this up with Nice Airport and their special assistance provider Samsic, who we understand the person photographed works for."
The airline apologised for the delay to flight EZY2122 from Nice to London Luton because of a "technical issue with the aircraft".
The statement released on Sunday said: "Passengers were provided with updates and refreshment vouchers during this time and the flight landed in London Luton last night.
"The safety and well-being of our passengers and crew is always easyJet's highest priority.
"EasyJet sincerely apologises for the delay and thanks passengers for their patience."
But Mrs Arkwright has penned an open letter to easyJet boss Dame Carolyn McCall, demanding she write personally to every person on the flight with an explanation.
"I should hardly need to tell you that customers should be treated with respect not disdain. It's nothing short of a shambles and a disgrace to British aviation," she wrote.