Montoya lapped the Jerez circuit in southern Spain with a best time of one minute 16.630 seconds, more than a second quicker than Brazilian Ricardo Zonta in a Toyota, and completed 51 laps.

"We've concentrated on trying a variety of different set ups and collecting initial data from the car," said Williams chief operations engineer Sam Michael.

"Naturally, we have encountered one or two teething problems, as you would expect with a new car, but Juan managed to complete some good mileage."

Williams unveiled the highly distinctive FW26, with an innovative front wing, in Valencia on Monday and gave it a limited shakedown on Tuesday at that circuit before heading south for a week of serious testing.

Michael said on Tuesday that Williams, runners-up to Ferrari last season after challenging strongly for the title, were "already miles in front of where we were last year".

Montoya's initial reaction was also positive.

"The car doesn't feel a lot different from last year's," he said after that first run in Valencia. "I think we have a potentially excellent car."

Renault, who launch their new car in Sicily later this month, also began their first test of the year at Jerez with France's Franck Montagny and Italian Jarno Trulli.

Brazilian Felipe Massa, who will race for Swiss-based Sauber this season after a year as Ferrari test driver, turned out for the Italian team once again alongside regular tester Luca Badoer.