Grove based Williams added that the car will be in a "heritage blue livery", presumably as a precursor to an official launch, which has yet to be announced.
The team, a poor ninth in the 2013 constructors' standings, said on Saturday that boss Sir Frank Williams was present in the factory race bay when the newly Mercedes-powered FW36 was fired up for the first time.
"2014 starts here," Williams announced.
Meanwhile, the second Red Bull team Toro Rosso, having already announced a Monday launch before the Jerez test, said its new STR9 is now clear to debut after passing the FIA's mandatory crash tests.
An image of the car's frontal test can be seen at https://pbs.twimg.com/media/BecS9zrCAAMYd_g.png:large
"A team truck will set off from the Faenza factory at the end of this week," Toro Rosso said on its website, "making the long trek to southern Spain, carrying not only the car but also all our hopes for the coming season."