The two-time Formula One racing world champion suffered a concussion when he hit the wall exiting Turn 3 shortly after midday at the Spanish track. He was taken to the circuit's medical centre and subsequently to hospital, where he was given precautionary CT and MRI scans that revealed he had not suffered any injuries.
"Fernando's accident was just one of those things that happens in testing," McLaren-Honda racing director Eric Boullier said.
"Fortunately, he's fine, but was concussed during the accident, which therefore required an overnight stay in hospital as a precaution. That's normal practice after a concussion. Inevitably, some media reports have sought to exaggerate the severity of the incident - it was just a normal testing accident."
Jenson Button had been due to take over from Alonso for the afternoon, but Boullier said the team opted not to attempt to repair the car in time to run again on the final day.
"While the car wasn't particularly badly damaged, it was enough of an impact to warrant quite a lengthy check of the gearbox and power unit systems," he explained. "Given the time needed to carry out such an analysis, we decided to bring the curtain down on our test a few hours earlier than anticipated.
"It's been a tough week for the whole team, but we'll be back, and pushing harder than ever, in Barcelona next week."
The crash brought to a close another difficult test for McLaren, as the team racked up just 104 laps in total over the four days at Barcelona. There are just four more days of pre-season testing, also at Barcelona, on February 26-March 1.