Williams's 2013 Formula 1 car will not be ready for the first winter test of the season and will make its debut at the second test on 19 February.
The team has announced that the FW35 will be unveiled at Barcelona on 19 February, the opening day of test two.
For the first test at Jerez on 5-8 February, Williams will run an updated version of its 2012 car.
"We will be running the FW34 (with 2013 livery) in Jerez to test a number of key development parts in preparation for the launch of the FW35," said a team statement.
Williams has retained Renault power for 2013, with Spanish Grand Prix winner Pastor Maldonado also staying on.
Former test driver Valtteri Bottas replaces Bruno Senna in the second car.
Last season the team took its first victory since 2004, but inconsistent form meant it only finished eighth in the constructors' championship.
2013 pre-season dates