Ernestina Mondragon was probably not surprised when she was pulled over by police after making an illegal U-turn in Dallas, Texas. She probably thought it was a fair cop when she was given an additional ticket for not having her driving licence.

But when the trainee officer fined her $204 for being a "non English-speaking driver" – a penalty later signed off by the officer's field-training instructor and sergeant – Mondragon fought back.

She contested the charge and it was dropped by the court. A subsequent investigation revealed that at least 39 drivers in the last three years have been ticketed for being "non English-speaking drivers".

Police officials in the Texas city of 1.3 million people believe the error originates in a menu on the computers used in patrol cars. Federal law requires that certain commercial lorry drivers speak English, but no such requirement applies to private motorists.

The Dallas police chief, David Kunkle, apologised at a press conference last week. "This is something I wish hadn't happened. It is an embarrassment."

Kunkle pledged to investigate the incidents, cancel pending charges and refund those who had already paid the fine.

"It sounds like a policy," Southern Methodist University law professor George Martinez told the Dallas Morning News. "Discrimination on the basis of language ability, and that's targeting Latinos, and so that sounds pretty serious to me."

In Dallas, 42% of the population is Hispanic and 43% speak a language other than English at home.

The US has no official language, although a movement has been afoot among conservatives to enshrine English in US law as the official tongue.