Reports from the US say a federal judge has ruled two Stabucks customers can pursue a lawsuit against the coffee giant for apparently under-filling its lattes.

They are accusing the chain of cheating patrons.

Reuters reports US District Judge Thelton Henderson said the California plaintiffs may seek damages from Starbucks, including for fraud and false advertising.

It is part of a proposed nationwide class-action suit.

Starbucks has been accused of overcharging customers by serving lattes that are 25% too small, based on a recipe it adopted in 2009 to save money on milk.

The plaintiffs claim Starbucks is using pitchers for heating milk with "fill to" lines that are too low.

Judge Henderson said it was "probable" that a significant portion of the public believe a hot drink should contain the exact number of fluid ounces advertised on the cup.

A Starbucks spokesman said the company believes the lawsuit is without merit, and is prepared to defend itself against the claims.