Takeaway behemoth Burger King will have to face court to defend claims it has cheated hungry customers with ads that inflate the size of its Whoppers, after a judge’s ruling.

Customers in the US have brought a class action lawsuit against Burger King, alleging its burgers appear 35 per cent bigger in advertisements than what buyers actually get.

The lawsuit, filed in Florida, alleges Whoppers are shown as having ingredients that “overflow the bun” and the meat is depicted as twice what is actually served up.

The US takeaway giant tried to have the case dismissed, but its bid was denied last week.

On Tuesday (US time), the takeaway giant hit back, saying customers’ claims of misleading ads were false.

“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,” the fast food chain said, according to Reuters.

The statement follows a decision in a Miami court last Friday. District judge Roy Altman ruled that Burger King must defend claims its depiction of Whoppers on in-store menu boards was misleading to reasonable customers, amounting to a breach of contract, Reuters said.

According to the court documents, the plaintiffs believe the Whoppers had once been “more fairly” advertised on Burger King’s website and on in-store menus.

However, in September 2017, Burger King “began to materially overstate the size of its burgers in its advertisements”, they allege.

The hungry customers say the size of the burger in the ads was bulked up by 35 per cent, with the beef patty doubling. But burger buyers didn’t get to bite into that actual increase.

Burger King labels lawsuit as “unreasonable”

Burger King, a unit of Restaurant Brands International, claims it wasn’t required to deliver burgers that look “exactly like the picture”.

But Judge Altman said it would be up to jurors to “tell us what reasonable people think”.

In its motion to dismiss the case, Burger King argued that the point of its advertisements was to make the food look as “appetising as possible”.

“That is hardly news; reasonable consumers viewing food advertising know it innately,” it argued, according to court documents.

“This lawsuit unreasonably pretends otherwise.”

In his decision made public last Friday (US time), Judge Altman also let customers pursue negligence-based and unjust enrichment claims.

He dismissed claims based on TV and online advertisements, as there was no evidence Burger King had promised a burger size or patty weight and then failed to deliver it.

Earlier efforts to mediate an outcome in the case have failed.

Elsewhere, Burger King rivals McDonald’s and Wendy’s are defending a similar lawsuit in New York. The plaintiffs’ lawyer there has cited Judge Altman’s opinion to justify letting that case continue.

Another takeaway chain, Taco Bell, was sued last month in the Brooklyn court for selling Crunchwraps and Mexican pizzas that allegedly contain only half as much filling as advertised.

Each lawsuit seeks at least $US5 million ($A7.7 million) in damages.