Six top US apparel brands that buy blue jeans from Mexican jeans manufacturer Grupo Navarra are calling for a free choice of union for workers at the company's factory in Tehuacan, Mexico.

Representatives from Abercrombie & Fitch, American Eagle Outfitters, Express, Gap Inc, Levi Strauss and Warnaco have signed a letter addressed to the Puebla State Secretary of Labour, urging authorities to set a "definitive date" for a union representation vote.

Workers are currently required to be members of an official union that is linked to Puebla state government and the PRI party that governs the state, according to workers' rights organisation Toronto-based Maquila Solidarity Network (MSN).

Its letter also askes for the vote to be "undertaken in accordance with applicable laws and in a manner that gives workers the ability to register their preference without fear or intimidation", said MSN.

"Holding the vote in an accessible and neutral location that protects the confidentiality of the workers would help ensure such an outcome," it continues.

Lynda Yanz, coordinator of MSN, said that the organisation appealed to the brands following reports that workers were under pressure not to support an independent union.

She said: "To make the situation even more complicated, another official union has also entered the picture and has been given free rein inside the factory to recruit members while the supporters of the independent union are being harassed and fired."