Williams have been forced to withdraw from the opening day of Formula One pre-season testing because their car is not ready.
The British team were due to be among the sport's 10 outfits to hit the track at Barcelona's Circuit de Catalunya on Monday for the opening winter running.
But in a blow to the team, which last year finished rooted to the foot of the constructors' championship, Claire Williams has revealed they will only be watching from the sidelines.
The Williams deputy team principal said: "Unfortunately, we have had to delay the start of our pre-season Barcelona test to Tuesday morning.
"We have had an incredibly busy winter at Grove getting the FW42 prepared for the season ahead and, despite everyone's best efforts, we need some more time before it will be ready to run.
"Our absolute priority is to always ensure we bring a car to the track that is the best that it can be and sometimes that takes longer than you'd anticipate or like. It's clearly not ideal, but equally it's not the end of the world.
"We will obviously have our work cut out to recover the time lost but we still have seven days of testing left and we will be maximising that time to prepare the car for the first race."
British rookie George Russell, who celebrated his 21st birthday on Friday, and the returning Robert Kubica will now be playing catch-up for a team bidding to bounce back from last season's miserable campaign.
The first pre-season takes place over four days before a second four-day test the following week. The new campaign starts in Melbourne on March 17.
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