Lack of staff and inadequate safety provisions in the workplace were leading to a high number of accidents at work, and it was not rare that around 30% of workers were signed off sick, the union said in a statement.

Reuters was not able to independently verify the union's claims and Tesla did not immediately respond to a request for comment.

Union representatives met workers at the factory gates, on the platforms of nearby stations and inside the factory on Monday handing out stickers stating "Together for safe and fair work at Tesla".

On Sunday night, Tesla managers invited their teams to a meeting with "free food and a surprise" to discuss IG Metall's presence on the site, stating: "We want to speak with you and your teams about the questionable methods and actual goals of IG Metall," according to a copy of the email seen by Reuters.

"The law gives all workers the right to organise in a union and stand openly for that at their workplace. That counts at Tesla in Brandenburg as well," Dirk Schulze of IG Metall said.

The union said it does not share specific membership numbers for companies as a matter of course, but that it has seen a steep rise in the number of new members at Tesla.

Reuters spoke to twelve workers at the factory on Monday.

While four said they were satisfied with working conditions, eight said pressure was too high, with some reporting high incidence of accidents and issues with receiving overtime pay.

Two workers said they were not allowed to speak to the media.

"Speed is not compatible with safety," said one 56-year-old worker from Poland, who declined to be named, adding there were too few workers to meet targets and that he would seek a new job next year if conditions did not improve.

(Reporting by Victoria Waldersee; Editing by Sharon Singleton)

By Victoria Waldersee