Sebastian Vettel crashed his Ferrari as McLaren proved quickest again on day two of Formula 1's second pre-season test in Barcelona.

Vettel was taken to the medical centre and declared unhurt by Ferrari after he went straight on at Turn Three and ploughed into the barriers due to a front-end mechanical problem.

The four-time world champion had already registered the third fastest time during his 40 laps on Wednesday morning.

But it was McLaren who topped the leaderboard for the second successive day, with Carlos Sainz fastest after teenage team-mate Lando Norris had gone quickest on Tuesday.

Mexico's Sergio Perez (Racing Point) registered the second fastest time of the day ahead of Vettel, then Alfa Romeo's Kimi Raikkonen and Romain Grosjean (Haas).

Ferrari, who later confirmed Vettel's crash was caused by a mechanical issue, impressed in the initial days of testing, with their car looking a step ahead of rivals Mercedes.

But Tuesday's session was marred by a problem with the cooling systems during Charles Leclerc's scheduled time at the wheel and Vettel managed less than half the laps of his main rivals.

Ferrari managed to send out Leclerc again before the end of the day, but only for the final 10 minutes.

Lewis Hamilton, ninth quickest, one place behind team-mate Valtteri Bottas, completed 102 laps as Mercedes continued to test their new aero parts.

Seven teams hit 100-plus laps for the day, with Sainz and Williams' Robert Kubica both leading the way with 130 and Raikkonen, Grosjean, Red Bull's Max Verstappen and Daniil Kvyat (Toro Rosso) also hitting the century mark.

There are two of the eight days of pre-season testing remaining and some of the big guns are expected to step things up on Thursday and Friday.

The opening round of the 2019 world championship is scheduled to start in Melbourne on March 17.