Pedro de la Rosa will drive the third McLaren for the first three races of the Formula One season.
De la Rosa has been a McLaren tester since 2003
McLaren said the Spaniard would be given the chance as he had "done the majority of the test work" on the team's new car over the winter.
The 33-year-old is hoping it will help him win back an F1 race seat in 2006.
"I've been guaranteed the seat for Australia, Malaysia and Bahrain. It is one of the final chances I will get," De la Rosa told Spanish paper As.
Austrian Alexander Wurz is McLaren's other test driver, but he has had problems fitting into the new MP4-20 and is yet to test it.
The team said they would decide who would drive for the rest of the season at a later date.
McLaren are able to take advantage of the rule that allows the bottom six teams from last year to run a third car in Friday practice at Grands Prix.
But having De la Rosa on hand to help with set-up and tyre-choice decisions could be invaluable as the team seek to dethrone Ferrari this season.
Ferrari, along with BAR, Renault and Williams, are not allowed to run a third car.
In theory, McLaren will be able to limit the mileage done by race drivers Kimi Raikkonen and Juan Pablo Montoya without sacrificing their ability to prepare their cars adequately for the race.
That could give them a crucial advantage in the light of new rules forcing drivers to use one engine for two entire race weekends, with severe penalties if a new engine is needed.
De la Rosa added: "For the next races they will decide whether it's Wurz or me. But at the moment I'm only thinking about those three races."
"I will try with all my strength, as I usually do," he said.
The Formula One season starts in Melbourne on 6 March.