Lewis Hamilton's pre-season preparations hit another bump in the road after the world champion spun on the second day of testing.

Hamilton's Mercedes team missed the majority of Friday's first session in Bahrain following a gearbox failure.

And, after team principal Toto Wolff said the sport's all-conquering constructor could ill-afford any more setbacks, Hamilton made an uncharacteristic mistake.

The Briton lost control of his car through the right-handed Turn 13, before sliding backwards and beaching his Mercedes in the gravel.

Hamilton's accident saw the session stopped as his Mercedes was taken back to the garage on a truck.

The 36-year-old, who is this season bidding to win an unprecedented eighth world championship, later returned to the circuit to complete 58 laps.

But he finished the day last but one on the time charts, 3.1 seconds off the pace, albeit having set his best lap on the slowest hard tyre compound.

With only three days of pre-season action – down from as many as eight in recent years – Hamilton will be back in his Mercedes for one final time on Sunday afternoon before he opens the defence of his title in the Gulf Kingdom on March 28.

Valtteri Bottas took over from team-mate Hamilton after the lunch break and provided a timely boost for the Silver Arrows by posting the quickest time of the test.

The Finnish driver edged out AlphaTauri's Pierre Gasly by 0.124 seconds, with Lance Stroll third for Aston Martin.

British driver Lando Norris continued McLaren's encouraging pre-season form by finishing fourth, three tenths off Bottas' pace.

Fernando Alonso is back on the grid this season following a two-year hiatus, and the double world champion posted an impressive 128 laps – the equivalent of more than two race distances – less than a month after he fractured his jaw in a road cycling accident.

There was trouble for Sebastian Vettel as a gearbox failure restricted the four-time world champion to only 10 laps.

"Unfortunately we did not have a productive morning," said Vettel, who has joined the British team after he was dropped by Ferrari.

"We had an issue with the gearbox and we had to change it and that cost us quite a bit of time.

"We did not get many laps in so this morning was not great, in terms of track time, but it is a long year and a long season ahead of us. It is crucial to settle in quickly but eventually we will all improve with more time on the track."