Dyson, the engineering company best known for its vacuum cleaners and fans, plans to spend £2bn developing a "radical" electric car.
The battery-powered vehicle is due to be launched in 2020.
Dyson says 400 staff have been working on the secret project for the past two years at its headquarters in Malmesbury, Wiltshire.
However, the car does not yet exist, with no prototype built, and a factory site is yet to be chosen.
Sir James declined to give further details of the project. "Competition for new technology in the automotive industry is fierce and we must do everything we can to keep the specifics of our vehicle confidential," he told staff in an email.
Important points that are undecided or secret include the firm's expected annual production total, the cost of the car, or its range or top speed.
Sir James said about £1bn would be spent on developing the car, with another £1bn on making the battery.