The former champions are the first team to state publicly that they will not have their 2013 car on track for the opening test.
Champions Red Bull, Ferrari, Mercedes and McLaren have all announced car launches before Jerez as have Sauber, Force India and Toro Rosso. Kimi Raikkonen's Lotus are also expected to test with the new car.
There are, however, minimal regulation changes from last year to this - with a radical overhaul scheduled for 2014 when a new power unit comes in - and Williams will be testing 2013 development parts on the FW34 in Jerez.
The old car will run in the 2013 livery.
Williams finished last season eighth overall and have a changed driver line-up this year with Finland's Valtteri Bottas replacing Brazilian Bruno Senna alongside Venezuelan Pastor Maldonado.
They have also lost chief operations engineer Mark Gillan, an important figure in the team's marked improvement on the track last year, since the end of the 2012 season.
Williams took their first victory in eight years in 2012 with Maldonado winning at the same Barcelona track where the new car will be launched.