A public relations officer linked to Walmart posed as a journalist at a press conference held by a labor group highlighting tough working conditions in the warehouses that supply big retailers.

Stephanie Harnett, a publicist working for Mercury Communications, which has been retained by Walmart to assist in its effort to open a new store in the Chinatown area of Los Angeles, claimed to be a student journalist called "Zoe Mitchell" when she turned up at the event on 6 June.

She then spoke to and recorded an interview with an activist from Warehouse Workers United, a group campaigning for worker improvements in the notoriously low wage industry where casual labor and poor health conditions are all too common.

The subterfuge only became apparent on Wednesday, when Harnett turned up at a different event and this time used her real name. She was spotted by members of WWU who recognised her and were stunned to see her handing out Mercury business cards with a completely different identity.

Walmart moved to distance itself from her actions on Thursday, and Mercury said neither it nor the retail giant had "approved, authorized or directed" her actions. It said she was no longer working for the firm.

WWU officials, who are backed by the Change to Win coalition of unions, are outraged at the stunt. They say Walmart – with its gigantic spending power – has a chance to improve things.

"Last week when Walmart had the chance to talk about real issues affecting Latino workers in Southern California it instead sent 'Zoe', a fake reporter. A spy. Our door is open. Walmart can change this industry and create thousands of good jobs and improve the quality of life in Southern California, but first it has to come out of hiding," the WWU said in a statement.

The press conference was to highlight a report by the National Employment Law Project which accused big box retailers, like Walmart, of driving down wages and work place safety standards in a supply chain of sub-contracted warehouse operators. The report, called Chain of Greed, singled out Walmart as a major offender. "Walmart sets the parameters for the working conditions in (warehouses)... but when things go wrong, it's the contractors that are blamed, while Walmart skirts responsibility for its actions and accountability for its influence over those engaged in its massive supply chain," the report stated.

For its part Mercury said that neither it nor Walmart had known about Harnett's plans to infiltrate the press conference as "Zoe" and gather information on participants. It also said that such an act would not happen again. "The action taken by Ms Harnett was in no way approved, authorized, or directed by Walmart or Mercury. Stephanie is a junior member of our team who made an immature decision. She showed very poor judgment and Mercury takes full responsibility. We are taking the necessary disciplinary actions. This is an isolated incident that has never happened before and will not happen again," Mercury said in a statement.

Warehouse worker Santos Castaneda, 25, one of two members of WWU who were speaking at the first press conference, is upset that he was duped by someone working indirectly for the company that he is campaigning against. "I never thought she was going to be a spy. I feel mad and disappointed," he said.

The "journalist" approached Castenada, 25, asked him to tell his story about low wages and tough conditions in his job.

The woman reporter introduced herself to Castaneda as a University of Southern California journalism student called Zoe Mitchell. "She said she was a story teller from the heart. She was very interested in warehouse workers and the working conditions," Castaneda said.

The pair spoke for up to half an hour and Mitchell recorded the whole interview, taking down details of Castaneda's biography and working history. "I explained how we are fighting for our rights," Castaneda said.

Walmart spokesman Steven Restivo told Gawker: "These actions were unacceptable, misleading and wrong. Our culture of integrity is a constant at Walmart and by not properly identifying herself, this individual's behavior was contrary to our values and the way we do business. We insist that all our vendors conduct themselves in a way that is transparent and honest and we will reinforce that expectation to help ensure this type of activity is not repeated."