Anna Turley, a Member of Parliament (MP) who took part in the group visit to the Shirebrook warehouse on Monday, said a woman who had brought them a tray of sandwiches had placed a camera and recording device on the floor underneath.

The bizarre incident was the latest in a series of public relations disasters for Sports Direct, a 450-store sportswear chain whose working conditions have been likened by media and politicians to those of a Victorian workhouse.

"I find it extraordinary what was reported this morning, especially for a company that has made declarations that it wants to improve its reputation and its image," business minister Greg Clark told parliament.

Sports Direct said its board had not authorized the use of a recording device and had no knowledge of the incident.

"The Board is disappointed that reporting of a possible recording device (the veracity of which has yet to be determined) has overshadowed the truly important issues that the visit should have focused on - the true working conditions and worker satisfaction at Shirebrook," it said in a statement.

Turley tweeted a picture which she said showed the tray of sandwiches placed on a stool in a room where the lawmakers had sat down for a private conversation at the end of their tour, and the camera on the floor under the stool.

"The poor woman who brought in a tray of sandwiches had to try & hide a camera under the table #bizarre," Turley tweeted.

Sky News, which filmed part of the lawmakers' surprise visit to the warehouse in Derbyshire, central England, showed footage of Turley confronting Gary Thompson, a Sports Direct spokesman, shortly after the incident.

"I've no idea, I've no knowledge of that," Thompson is heard saying in the video. "I don't know what's happened there."

Turley responds: "It happened in front of my eyes."

"All the trust that was built up today ... was undone with that, because how can we trust anything that you say or anything that we've seen when you ... get the sandwich lady to sneak a camera in?"

Thompson responds: "Well look, I don't know about the camera, but you've been around the whole site for four hours, you've seen everything, isn't that the thing that matters to form a balanced view on the warehouse and the staff?"

Sports Direct's billionaire founder Mike Ashley, who was rebuked by lawmakers over working practices at his business in July, responded to the incident in typically combative fashion.

"I stand firmly behind the people of Sports Direct, who through no fault of their own have been made a political football by MPs and unions," he said in a statement.

Sports Direct cut its profit forecast twice last month and has seen its share price slump this year. It has admitted to shortcomings in its working practices and said it was addressing them.

(Editing by Stephen Addison)

By Estelle Shirbon