Williams will miss the first day of Formula One's pre-season testing in Spain on Monday because they have yet to finish their car, the former champions said on Sunday.
The delays had already forced the British team, last overall in 2018 with one of the worst-handling cars they have ever built, to cancel a filming day at Barcelona's Circuit de Catalunya on Saturday.
"Unfortunately, we have had to delay the start of our pre-season Barcelona test to Tuesday morning," deputy principal Claire Williams said in a statement.
"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."
Williams scored just seven points from 21 races last year, despite having the same engine as champions Mercedes in the back of the car.
They have a changed line-up for the new season starting in Australia on March 17, with British rookie and Formula Two champion George Russell partnering Poland's Robert Kubica, returning from a near-fatal rally accident in 2011.
Russell will be first to drive the car on Tuesday, all being well, with Kubica on Wednesday and both on Thursday. Canadian reserve Nicholas Latifi will also be in the car on Tuesday afternoon and Wednesday morning.
"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," said Williams.
Formula One teams always operate to tight margins in designing and building cars, seeking to maximise development time, and this year has seen significant new aerodynamic regulations come into play.
Other teams, including Mercedes and Red Bull, have had their new car running on track already for shakedown sessions, however.
Renault, who had feared they too would have to scrap a filming day on Saturday as they scrambled to assemble the car, made it in time with Australian Daniel Ricciardo putting in the first laps.