A spokesman said the use of the king's image must be approved by the royal family, but they had not been asked.
On the website, Philippe is represented by a cartoon.
"We disapprove of this approach," royal spokesman Pierre Emmanuel de Bauw said.
"Since it is for commercial purposes, we would not have given our authorisation."
On the ad website, if a user chooses Burger King no more prompts appear. If they choose Philippe, they get asked twice whether they are sure - and the second time, the only option is "no".
Votes are open until 19 June. The chain will open its first outlet in Belgium at the end of June.
A Burger King representative told the news agency EFE it had not received any communication from the monarchy.
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