Pizzonia had a torrid time at Jaguar last year

The Brazilian was a test driver for the team in 2002 before moving to Jaguar in 2003 for an unsuccessful racing stint.

Pizzonia, who was dropped by Jaguar after last year's British Grand Prix, said: "It feels great to be back working with everyone at Williams.

"My long-term future in Formula is as a race driver, but my aim for this year is to do everything I can as a test driver to help the team."

Williams technical director Patrick Head said last year that he had always been impressed by Pizzonia and did not know why he had struggled with Jaguar.

Pizzonia has already been involved in testing the new Williams FW26 this winter, and the team said he would carry out a "crucial role" for the rest of the year.

He will join Spaniard Marc Gene as test driver for the team.