Lewis Hamilton insists Mercedes remain Formula One's team to beat after an ominous opening day of pre-season testing in Barcelona.

While the sport has undergone a radical overhaul of its regulations over the winter in a bid to spice up the show, it was a familiar driver and team at the top of the order on Monday night.

Hamilton and Valtteri Bottas, the Finn who has replaced world champion Nico Rosberg following his shock retirement, completed more mileage than any other team.

And Mercedes, winners of the past three driver and constructors' championships, boasted impressive speed, too, with Hamilton fastest from Ferrari's Sebastian Vettel.

"I still think we're the team to beat," said Hamilton. "We have a lot of work ahead of us to do, the same as every single team on the grid.

"Right now we're just focusing on our own job to do the very best that we can, but it's been a good day, a really positive day for the team.

"The car looks fantastic and it feels great. We did lots of laps and collected loads of information today, so we can try to improve the car as we move forward."

Hamilton, bidding to become the first British driver to win a fourth world title, will be back in his Mercedes on the second morning of this four-day test.

Reflecting on being earmarked as the favourite to reclaim the championship he lost to Rosberg last season, the confident Briton added: "Firstly, people always want to big you up at the beginning of the season. Secondly, we are the world champions, and thirdly our car looks a thousand times better than everyone else's so that would be expected."

Although it was business as usual for Mercedes, McLaren endured a troubled start to their new campaign. Fernando Alonso missed most of the day following a problem with his Honda engine.

Hamilton believes Mercedes are still the team to beat (Getty)

There was trouble for Red Bull's Daniel Ricciardo, too. He suffered a gearbox issue before a battery failure curtailed his running later in the day.

But there were promising signs for Ferrari. The famous Italian team are bidding to bounce back from a disappointing campaign in which they failed to register a single victory and Vettel, the four-time world champion, finished second only to Hamilton with an impressive 128 laps under his belt.

Hamilton and Bottas will share the testing duties for Mercedes on Tuesday while Jolyon Palmer, the only other British driver on the grid this season following Jenson Button's retirement, will get his first taste of the new Renault.