A US judge has ruled that Burger King must face a lawsuit that alleges the company distorts the size of its Whopper burger.

The ruling came in a class action brought by Burger King customers, who claim the fast-food franchise is lying about the size of its Whoppers.

US District Judge Roy Altman in Miami said Burger King must defend against a claim that its depiction of Whoppers on in-store menu boards misleads reasonable customers, which amounts to a breach of contract.

In Australia, Hungry Jacks operates as a franchise of Burger King.

The customers who brought the class action suit say Burger King’s promotional images show ingredients that “overflow over the bun”, making it appear as if the burgers are 35 per cent larger and contain more than double the meat than the chain actually serves.

Burger King said it wasn’t required to deliver burgers that look “exactly like the picture”.

However, the judge said it was up to jurors to “tell us what reasonable people think”.

“The plaintiffs’ claims are false,” Burger King said in a statement.

“The flame-grilled beef patties portrayed in our advertising are the same patties used in the millions of Whopper sandwiches we serve to guests nationwide.”

McDonald’s and Wendy’s are also defending a similar class-action in a federal court in Brooklyn, NY, which alleges that the burgers from the two restaurant chains are at least 15 percent larger in marketing materials.