By Hyunjoo Jin and Tiyashi Datta

(Reuters) - Tesla Inc Chief Executive Elon Musk has asked employees to return to the office or leave the company, according to an email sent to employees and seen by Reuters.

"Everyone at Tesla is required to spend a minimum of 40 hours in the office per week," Musk wrote in the email sent on Tuesday night.

"If you don't show up, we will assume you have resigned."

"The more senior you are, the more visible must be your presence,” Musk wrote. “That is why I lived in the factory so much — so that those on the line could see me working alongside them. If I had not done that, Tesla would long ago have gone bankrupt.”

Two sources confirmed the authenticity of the email reviewed by Reuters. Tesla did not respond to a request for comment.

Major tech firms in Silicon Valley do not require workers to return to the office full-time, in the face of resistance from some workers and a resurgence of coronavirus cases in California.

Tesla has moved its headquarters to Austin, Texas, but has its engineering base and one of its factories in the San Francisco Bay area.

"There are of course companies that don't require this, but when was the last time they shipped a great new product? It's been a while," Musk wrote in the email.

"Tesla has and will create and actually manufacture the most exciting and meaningful products of any company on Earth. This will not happen by phoning it in."

One of Musk's Twitter followers posted another email that Musk apparently sent to executives asking them to work in the office for at least 40 hours per week or "depart Tesla."

In response to this tweet, the billionaire, who has agreed to take Twitter Inc private in a $44 billion deal, said, "They should pretend to work somewhere else."

Some Tesla workers expressed displeasure over Musk's latest comments in posts they placed on the anonymous app Blind, which requires users to sign up using company email as proof of employment at firms.

"If there’s a mass exodus, how would Tesla finish projects? I don’t think investors would be happy about that," one Tesla employee wrote.

"Waiting for him to backpedal real quick," another worker posted.

A California-based workers advocacy group assailed Musk's return to office plan.

"Employers including the state government are finding that mandating a return of all employees is a recipe for outbreaks," Stephen Knight, executive director at Worksafe, wrote in an emailed statement to Reuters.