McLaren endured yet another disastrous start to testing after Fernando Alonso broke down inside the first hour on Friday.
The double world champion was completing only his seventh lap when he stopped at turn seven of Barcelona's Circuit de Catalunya.
McLaren are changing Alonso's engine as they investigate the cause of his car's failure, but the Spaniard is set to lose further valuable track time.
McLaren's latest issue marks the third day of this week's concluding four-day test in Spain in which the British team have encountered problems.
The new Formula One campaign gets under way in Melbourne in a fortnight's time.
McLaren endured a troubled start to Formula One's concluding pre-season test after Stoffel Vandoorne broke down twice inside the opening hour.
Vandoorne had to be pushed back to the garage by his McLaren mechanics at Barcelona's Circuit de Catalunya when he stopped in the pit lane after completing one lap.
The Belgian returned to the track moments later but then broke down again, this time on the entrance to the pits, and was wheeled back by his McLaren team.
McLaren said an electrical issue was at cause for both failures, and were optimistic Vandoorne would be back in his car before the lunch break.
But the British team will be frustrated at missing the majority of the morning session with only three further days of testing remaining before the new campaign gets under way in Melbourne on March 25.
A faulty wheel nut cost Fernando Alonso valuable track time on the first day at Barcelona last week, while Vandoorne did not complete any running last Tuesday afternoon following mechanical gremlins.
McLaren, who finished last but one in the constructors' championship last year, expect to be back at the sharp end of the grid in 2018 after ending their nightmare relationship with engine partner Honda and teaming up with Renault.
Mercedes' Valtteri Bottas led the way from Ferrari driver Sebastian Vettel on the opening morning. Bottas and Vettel were separated by just one tenth of a second with Red Bull's Max Verstappen third in the order.
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McLaren's F1 team have announced that Fernando Alonso will race for the team in 2018.
Alonso's contract had been due to expire at the end of this season. The Spaniard's Formula One future was up in the air but he has now committed to the Woking-based side for at least another year.
Alonso told McLaren's official website: “It’s fantastic to be able to continue my relationship with everybody at McLaren.
"It was always where my heart was telling me to stay, and I really feel at home here.
"This is a fantastic team, full of incredible people, with a warmth and friendliness that I’ve never experienced elsewhere in Formula 1. I’m incredibly happy to be racing here."
“Just as important, McLaren has the technical resource and financial strength to be able to very quickly win races and world championships in F1.
"Although the last few years have not been easy, we have never forgotten how to win, and I believe we can achieve that again soon."
McLaren's woeful pre-season continued on the final day of the Barcelona test as Fernando Alonso's MCL32 twice ground to a halt at the Circuit de Catalunya.
The former world champion's team and engine suppliers Honda have been plagued by problems which showed no signs of getting any better, with the first race of the season in Australia just over two weeks away.
Alonso's car stopped twice on the track on Friday's morning session to add to the two red flags on Thursday as the team were again robbed of valuable track time ahead of the Melbourne race.
The first stoppage came just over halfway into the morning session, with Alonso's car eventually taking its now customary place on the back of the recovery truck after a short break.
But when Alonso returned to the track with a little over an hour of the session left it stopped again almost straight away.
Mercedes are considering Fernando Alonso as a replacement for retiring world champion Nico Rosberg.
But two-time championship winner Alonso is about to enter the final year of his three-year deal with McLaren, meaning Mercedes may have to look elsewhere.
"You have to consider Fernando," Mercedes boss Toto Wolff told Sky Sports' F1 Special: Nico The Brave show.
"He is a driver I respect a lot. He combines talent, speed and experience. It's all there.
"But he is in a contract with McLaren-Honda at the moment and we just need to weigh all the other options up."
Mercedes face a race against time to bring in a new driver alongside Lewis Hamilton, with pre-season testing due to begin in February.
Rosberg made his surprise announcement last week, just five days after sealing the drivers' championship.
Fernando Alonso has been ruled out of the Bahrain Grand Prix amid concerns he could suffer a punctured lung.
The 34-year-old - who sustained fractured ribs at last weekend's Australian Grand Prix in one of the most spectacular Formula One smashes in recent times - failed a medical test at Bahrain's Sakhir Circuit on Thursday, and his participation at the next race in China in a fortnight's time is also under threat.
Belgian's Stoffel Vandoorne, the untested McLaren reserve driver, has jetted in from Japan to deputise for the two-time world champion.
Fernando Alonso says he is fortunate to be alive after surviving one of the most spectacular crashes in recent Formula One memory.
While Nico Rosberg finished ahead of Lewis Hamilton and Sebastian Vettel to win the opening race of an incident-packed Australian Grand Prix, the season-opener will be remembered for Alonso's terrifying 200mph smash.
After he careered into the back of Esteban Gutierrez under braking for turn three, Alonso was merely a passenger as he slammed into the wall before barrel-rolling through the air twice.
When Alonso's car finally came to a standstill, the Spaniard was upside down, missing all four tyres and much of his McLaren bodywork littered the circuit. Miraculously, the two-time world champion crawled out of his car before limping away from the wreckage.
"I am lucky to be here and thankful to be here," said Alonso. "It was a scary moment and a scary crash.
"When I stopped I saw a little space to get out of the car and I went out quickly just to make sure that my mum, who was watching the race on TV at home, could see that I was okay."
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