Melania Trump - wife of Republican presidential nominee Donald Trump and a Slovenian immigrant - worked illegally as a model in the United States, according to the Associated Press.
Ms Trump arrived in the US on a visitor's visa on August 27, 1996, and waited seven weeks before receiving her work permit.
Using evidence from Ms Trump's modelling agency, AP concluded she earned more than $20,000 for work over that period - including jobs at Fitness magazine and the Bergdorf Goodman department store.
It is illegal to work on a visitor's visa in the United States.
It comes as a blow to Mr Trump, who has insisted his wife followed US immigration law, and built his campaign on cracking down on illegal immigration.
"I am pleased to enclose a letter from my immigration attorney which states that, with 100% certainty, I correctly went through the legal process when arriving in the USA," Ms Trump tweeted in September.
The Trump campaign has refused to comment on the allegations.