Jaguar have freed Mark Webber to test for Williams after the season-ending Brazilian Grand Prix.
Webber was due to join Williams at the end of the year but Jaguar have agreed to release him early.
A Jaguar statement: "In order continue to support Mark Webber, he will be released to start his winter test programme with his 2005 team."
Jaguar's future is uncertain after owners Ford decided to pull out of Formula One.