Williams were forced to abandon testing their new F1 car in Barcelona a day early because of a front-wing problem that could not be fixed on site.
Nakajima crashed while testing the new Williams
The problem caused Japanese driver Kazuki Nakajima to crash into the barriers on the first day on Friday.
Nakajima and Germany's Nico Rosberg resumed on Saturday but Williams decided the issue could only be rectified back in the factory.
"We could not implement an appropriate short-term solution," a spokesman said.
"We had a reliability issue with the front wing at Barcelona.
"We decided to return to the factory to address it in time for the Jerez test in just over a week's time."
Former champions Williams had recorded some impressive times with the new Toyota-powered FW30 car in previous tests in Spain this month.
Toro Rosso drivers Sebastian Vettel and Sebastien Bourdais led Saturday's timesheets at the Circuit de Catalunya after the Ferrari-powered team decided to simulate qualifying conditions with the cars running on a low fuel loads.
McLaren's Lewis Hamilton, last season's overall runner-up, was third quickest despite the Briton's afternoon ending early due to a problem with the transmission.
Honda's new car was again at the slow end of the timesheets, with Briton Jenson Button completing 58 laps with a best effort more than two seconds off Vettel's fastest.