The test at the Circuit de Catalunya will take place behind closed doors, without media access, and was due to be the first time all teams - now up to 11 with debutants Cadillac - had their 2026 cars on the track together.
The 2026 season marks the start of a new engine era with an overhaul of the technical regulations.
"Atlassian Williams F1 Team has taken the decision not to participate in next week's shakedown test in Barcelona following delays in the FW48 programme as we continue to push for maximum car performance," the team said in a statement.
"The team will instead conduct a series of tests including a VTT (Virtual Test Track) programme next week with the 2026 car to prepare for the first official test in Bahrain and the first race of the season in Melbourne.
"We are looking forward to getting on track in the coming weeks and want to thank all our fans for your continued support – there is a lot to look forward to together in 2026."
Williams, who have Spaniard Carlos Sainz and Thai Alex Albon as drivers, did not specify the exact cause of the delay but some media reports mentioned the FIA crash tests required to participate in testing.
Alpine driver Franco Colapinto, who has a long-term contract with Williams after starting his F1 career there, felt it was a setback for the team with limited track time available.
"If you miss Barcelona, the first three days of Bahrain will be just trying to get the car to work and understand," said the Argentine at the launch of his team's new car in Barcelona. "It's very important to get all the mileage you can and get all the knowledge possible."
Alpine, Ferrari, Cadillac and Audi have all run their new cars for limited mileage filming days and apart from Williams all the teams plan to be in Barcelona.
Alpine technical director David Sanchez said it was important to maximise the time available with so much change.
"You want the car to be on track as early as possible so you find the problems to fix and you give yourself time to fix it," he said.
Williams, who also missed two days of the opening pre-season test ahead of the 2019 campaign, finished in fifth place in the team standings last season -- their best since 2017.
The season begins with the Australian Grand Prix in Melbourne on March 8.
