Fernando Alonso was involved in a big crash on the opening morning of winter testing in Barcelona.
The right rear wheel came off the Spaniard's MCL33 in the final corner, pitching the two-time world champion into a spin and leaving him in the gravel trap.
McLaren are investigating the cause of the crash but have confirmed they expect to be off track for at least a couple of hours.
Alonso had completed only five laps before his accident, but he was able to climb out before inspecting the rear of his car.
"It appears that his right rear wheel came off which caused the crash," Sky
Sports' Craig Slater reported from the scene.
"McLaren are investigating the cause. It could be that it was a loose wheel nut or it could be some kind of failure in the wheel structure itself.
"This is a very fast corner, you are coming around here at around 160mph. Not the start McLaren would have wanted and Fernando Alonso is back in the garage.
"He came out of the car and closely examined what had gone on, he was staring at the wheel trying to determine why it had come off."
McLaren had been hoping to complete 100 laps per day during testing at the Circuit de Catalunya following their switch to Renault power for 2018.
Their 2017 testing proved a disaster in their final season with Honda, completing just 425 laps across the two tests and just 29 on the opening day.
The Sky Sports F1 Digital team will be providing live commentary from dawn until dusk for every day of the test while Sky Sports News will also deliver live updates from trackside.