The Spaniard had managed just 20 laps on another troubled morning for the struggling McLaren-Honda partnership when he crashed into the wall between turns three and four at the Circuit de Catalunya.
“Fernando’s accident was just one of those things that happens in testing. Fortunately, he’s fine, but was concussed during the accident, which therefore required an overnight stay in hospital as a precaution,” said McLaren’s racing director Eric Boullier.
Alonso’s agent Luis Garcia Abad added that the 33-year-old would remain in hospital under observation for the next 24 to 48 hours.
The crash brought McLaren’s day to an end to round off a disastrous test that has left them well behind every other team on the grid in terms of mileage in testing with just 203 laps completed under a month until the first Grand Prix of the new season.
“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,” added Boullier.
“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 other Spanish driver on show, Carlos Sainz Junior, also crashed at the same part of the track where Alonso went off, but was able to walk away from a check-up at the medical centre unharmed.
Lotus were again fastest as Frenchman Romain Grosjean followed teammate Pastor Maldonado’s example on days one and three by posting the best time of the week in 1min 24.067sec on the supersoft tyres.
However, the day arguably belonged to defending world champions Mercedes as Nico Rosberg completed another mammoth 131-lap run and was under three tenths of a second behind Grosjean on the slower medium tyre compound.
“Looking at the times it’s of course still not clear where we stand and we’re keeping a close eye on the competition, but we’re confident that we have a good car at least,” said a modest Rosberg.
Daniil Kyvat was third quickest for Red Bull, whilst Sebastian Vettel was down in seventh fastest during his 76 laps in the Ferrari.
The teams will return to the track in the Catalan capital for a final four-day test from Thursday to finalise preparations before the first race of the season at the Australian Grand Prix in Melbourne on March 15.
Results from day four of the second pre-season test in Barcelona:
1. Romain Grosjean (FRA/Lotus) 1:24.067 (111 laps)
2. Nico Rosberg (GER/Mercedes) 1:24.321 (131 laps)
3. Daniil Kyvat (RUS/Red Bull) 1:24.941 (104 laps)
4. Felipe Nasr (BRA/Sauber) 1:24.956 (73 laps)
5. Valtteri Bottas (FIN/Williams) 1:25.345 (129 laps)
6. Carlos Sainz Junior (ESP/Toro Rosso) 1:25.604 (88 laps)
7. Sebastian Vettel (GER/Ferrari) 1:26.312 (76 laps)
8. Nico Hulkenberg (GER/Force India) 1:26.591 (36 laps)
9. Fernando Alonso (ESP/McLaren) 1:27.956 (20 laps)