Workers at the Cadbury World visitor centre are staging a 24-hour strike in a dispute over pay.
The strike is one of three planned over the next three weeks
Ten staff at the site in Birmingham want parity with colleagues in the factory shop immediately next door, who they say are paid £2 an hour more.
The Transport and General Workers Union (T&G) said the walkout by workers based in the centre's shop in Bournville will disrupt the popular visitor attraction.
Cadbury World said it did not expect its visitors' enjoyment to be spoilt.
T&G national officer Brian Revell said: "Quite what Willy Wonka would make of this I don't know.
"The Cadbury managers have been obstructive throughout and refused to negotiate.
"Instead of a tour of a fantasy world this Thursday, people will be faced by the reality of a picket line.
"We don't want to spoil a day out but Cadbury have really left us with no other alternative."
The union says the company is comparing the pay rates to those at Alton Towers 40 miles away.
A Cadbury World spokesman said: "Cadbury World has offered to take this dispute to Acas but, unfortunately, this one-day strike has gone ahead despite that offer.
"However, Cadbury World remains open to the public and we do not expect visitors' enjoyment to be disrupted in any way."
The spokesman added that workers at the visitor centre do a different job from factory employees.
"It is simply not appropriate to compare the pay of Cadbury Word staff and those within our factory.
"Because Cadbury World is a stand-alone operation we need to generate our own profits to reinvest back into the attraction to ensure it retains its popularity.
"Compared to other leisure venues in the region our staff are some of the highest paid."
The 24 hour-strike, which started at 0700 BST, is one of three planned over the next three weeks.