“We are writing to express our deep concern about reports that your company practices a double standard toward American workers when it comes to respecting their labor rights,” according to the letter to René Obermann obtained by financial weekly Manager Magazin said on Tuesday.

The letter goes on to cite a December 2009 report by non-profit advocacy organisation American Rights at Work, which allegedly documented a systematic campaign by T-Mobile USA and parent company Deutsche Telekom to prevent US employees from forming unions.

“The company respects workers’ rights in Germany, where it cooperates closely with unions, but mistreats workers in the United States and interferes with their right to organize,” a press statement by the organisation said when the study was released.

The letter from all 26 politicians on the US congressional committee insisted that Obermann ensure workers are allowed to exercise their rights without interference, and inform them of his actions to see this through.

“When this freedom is curtailed, our economy suffers,” the letter read.

According to Manager Magazin, US union Communications Workers of America (CWA) is already working on filing a lawsuit against Deutsche Telekom through the international Organisation for Economic Co-operation and Development (OECD). Massachusetts Senator John Kerry has also allegedly sent a letter to Obermann about union complaints, the magazine said.

The Bonn-based company responded to the most recent pressure by telling Manager Magazin that labour laws are different in every country.

“In every country we deal with our workers fairly and correctly,” a spokesperson said, adding that US employees were free to form unions, but not to gather on T-Mobile USA property.

Meanwhile on Monday Obermann defended the company’s poor performance in the United States, admitting that the company wasn’t “fully satisfied” with growth, but insisting it was committed to its operations there, according to the Wall Street Journal.