Mercedes banked just six laps in the first session of Formula 1 pre-season testing in Bahrain after striking mechanical problems.

Valtteri Bottas took the wheel at the start of the day, completing an aero rake run before suffering gearbox dramas.

That saw the Finn spend much of the pre-lunch running in the garage as the team worked to replace the gearbox and understand the problem.

He was able to return to the track inside the last hour before the interval, only for a red flag to end the session early when Charles Leclerc suffered a misfire.

“We weren’t very fast this morning because we did one lap with an aero rake,” explained team boss Toto Wolff.

“It wasn’t a good start because we had a gearbox issue that came out of nowhere and that we haven’t been ready to identify and understand.”

Bottas himself added: “So, first day of testing obviously not quite ideal.

“For me in the morning, we had an issue with the gearbox on the first lap, took quite a bit of time to fix it but we got it back on track but only for a few laps.

“Hopefully over the next few days we can recover some of the mileage we lost today.

“It’s super windy here, so I think the afternoon running has been a bit compromised as well for Lewis [Hamilton, team-mate].

“Anyway, looking forward to efficient next two days and to actually discover how the W12 behaves.”

The gearbox problem follows an off-season that was not trouble-free.

Speaking last month, Mercedes engine boss Hywel Thomas admitted the team had “some issues” with its 2021-spec power supplies.

Only three days of testing has placed an emphasis on trouble-free running.

“I hope, if we are able to have a smoother ride from here onwards then I think we can recover,” Wolff said of the early set back.

“If we have more stumbling blocks then with three days there is not a lot you can do.”

Hamilton took over driving duties in the W12 following the lunch break, clocking up 42 laps.

While an improvement on what his team-mate had managed, it left Mercedes with a combined total of just 48 laps for the day.

That compared to 139 from Max Verstappen, who completed the day solo for Red Bull as he topped the day’s running.

Bottas ended the day at the foot of the timesheets with a 1:36.850s, while Hamilton recorded a 1:32.912s to finish the day 10th fastest, 2.3s off Verstappen’s ultimate pace.