The Tesla chief executive waded into the future of work debate by telling staff at the electric carmaker that they must return to the office for at least 40 hours a week or leave the company, according to an email seen by Reuters.

The IG Metall union in Berlin-Brandenburg-Sachsen, where Tesla's plant is located, said it would support any employee who opposed Musk's ultimatum. Tesla employs around 4,000 people in Germany and plans to expand the workforce to 12,000.

"Whoever does not agree with such one-sided demands and wants to stand against them has the power of unions behind them in Germany, as per law," Birgit Dietze, the district leader for IG Metall in Berlin-Brandenburg-Sachsen, said.

Employees at Tesla's plant in Gruenheide, Germany, elected 19 people to its first workers' council in February, setting the plant apart from others run by the carmaker in the United States and elsewhere without union representation, which Musk has fiercely resisted.

Some of the workers are part of IG Metall which represents workers across automotive companies and other industrial sectors.