BRUSSELS (Reuters) - Burger King is in trouble with Belgium's monarchy over an advertising campaign asking Belgians to vote online to "crown" the global fast-food giant the true ruler of the country where the U.S. brand will launch next month.

Representatives of Belgium's King Philippe on Monday asked the local unit of Burger King, owned by Restaurant Brands International , to explain itself.

"We told them that we were not happy with them using an image of the king in their campaign," palace spokesman Pierre-Emmanuel De Bauw told Reuters, adding that the monarch's image -- he appears in cartoon form -- could not be used for commerce.

Shana Van den Broeck, a spokeswoman for Burger Brands Belgium, said that the company is considering whether to make changes to the advertising.

"We are deliberating on how to proceed," she said. "Should we make a change to our campaign we would communicate that."

The animated advert, noting that King Philippe was crowned in 2013, announces the brand's launch in Belgium this month and asks: "Two Kings. One crown. Who will rule? Vote now ... "

Anyone clicking to vote for the 57-year-old monarch then faces a series of questions such as: "Are you sure ... ? He won't cook you fries."

The spoof poll may have touched a nerve in Brussels. In 1950 Belgians held a real referendum on a proposal to abolish the monarchy in light of the role of King Philippe's grandfather, Leopold III, during Nazi occupation. Leopold was forced to abdicate in favor of his son, Philippe's uncle.

(Writing by Alastair Macdonald; Editing by David Goodman)