Butlin's has been branded "racist" over a wrestling match which encouraged families to boo a Muslim "baddie".
Parents said they were "horrified" at the evening's entertainment at the Skegness resort over the weekend.
One father, Christian Ceriso, took to Twitter to blast what he called a "hate-filled ten minutes of everything wrong on racial stereotypes".
The company director from Newcastle added he felt he had been "dropped into the middle of a Britain First rally".
Butlin's later apologised and told ITV News in a statement: "We are investigating and apologise for the distress caused.
"Saturday's wrestling show was a deviation from the agreed content, and we are conducting a full review with our third-party supplier as to why this happened.
"We would like to assure guests that this was a very unfortunate one-off which we find completely unacceptable, and is totally contrary to Butlin's values. It will not happen again."
Brian Dixon, managing director of All-Star Wrestling who organised the entertainment, said: "Saturday's wrestling show at Butlin's Skegness was different to the agreed content, and Butlin's were not made aware of the change.
"We regret this clear error of judgment, as well as the obvious offence and upset caused.
"We wish to apologise to Butlin's and their guests, and stress that this will not happen again."