FW35 will only be unveiled at Barcelona on 19 February.
Jan.18 (GMM) Williams will not launch its 2013 car until after the first winter test.
That is the second of three winter tests in Spain next month, and two weeks after most other teams get their 2013 single seaters up and running at Jerez at the beginning of February.
"For the earlier Jerez test," the Williams statement said, "the team will run the 2012 FW34 in order to test a number of key development parts in preparation for the launch of the FW35.
"The FW34 will display 2013 livery."