Formula 1 drivers gave a cautiously positive reaction to this year's new cars at the end of the first day of pre-season testing in Spain on Monday.

Mercedes driver George Russell said: "The cars looked really cool out on the circuit and that's positive for our sport.

"They are quite different for us as drivers but, once you get your head around it, driving them is quite intuitive.

"It's enjoyable being behind the wheel and there's a lot for fans to look forward to with these new regulations."

The Briton's team-mate Kimi Antonelli described his car as "very nice to drive", Racing Bulls' Liam Lawson said "there is a lot more we can do as drivers to make a difference" and Audi's Gabriel Bortoleto said the new engines felt "very cool".

The drivers were speaking in pool interviews provided by rights holder F1 and governing body the FIA, or in team statements.

Teams are holding a private test at the Circuit de Barcelona-Catalunya, to which independent media have not been given access.

Secrecy was so intense that a number of journalists who were trying to observe the new cars from outside the circuit reported being moved on by security guards despite being on public land.

The decision was made to run the test in private as a result of the severe reliability problems a number of teams suffered when the engine rules were last changed in 2014.

This year, the cars are all new - with changes to the regulations governing chassis, engines, tyres and the mandating of carbon-neutral sustainable fuel.

The engines are the biggest change, with 50% of the power now provided by the electrical part of the hybrid engines, and energy management expected to be a major feature of the sport in 2026.

Seven of the 11 teams were running on Monday - Red Bull, Mercedes, Racing Bulls, Haas, Alpine and the new Audi and Cadillac teams.

McLaren said last week that they would not take their car on to the track until after day one as a result of trying to maximise design time. Ferrari, after an initial test at their factory last week, had also announced they would not run on Monday.

Aston Martin have said their new car will not be ready until Thursday, and Williams are missing the test entirely after delays to their design and production programme.

No times were issued, and all teams ran into at least some technical issues as they learned about their new cars.

Rival teams were impressed by the amount of mileage the two Red Bull teams managed considering it was the first full day of running with their new in-house engine, which has been developed in conjunction with new partner Ford.

Before the test started, McLaren and Red Bull revealed images of their 2026 cars for the first time.

McLaren, who won the drivers' and constructors' championship double last year for the first time since 1998, showed the car in the one-off testing livery it will run in this week.

Red Bull showed studio shots of their car, carefully chosen to disguise key design features.

Russell added: "We are pleased with our day, but I've also been impressed by several other teams.

"The Red Bull power unit has completed a lot of laps which, given that it's their first engine they've built, means they've clearly done a good job.

"Haas also managed a similar amount of running to ourselves, so the Ferrari power unit has also put together plenty of mileage. It's not quite how it was in 2014! The sport has evolved so much since then and the level, in every single aspect, is so high now."

Audi have taken over the Sauber team for the German car company's first entry into F1. They suffered a reliability issue that prevented Bortoleto driving in the afternoon.

Team principal Jonathan Wheatley said: "We had a technical issue, spotted it and decided to switch the car off. Plenty of testing this year and wanted to really understand the problems, see what the weather conditions are and decide whether we run tomorrow."

Rain is forecast for Tuesday's second day of the test, during which teams are permitted to run on a maximum of three of the available days.

Bortoleto said: "The cars are very different. I feel these are going to be slower but it's very cool to have the power unit being 50% electric - you go out of the corner and you have so much speed being deployed and you can see how strong the engine is.

"You need to adapt but it's still a racing car."

Racing Bulls' Lawson said: "[The car is] very different to drive and I haven't got my head around it fully yet. We will keep learning. It feels like there is a lot more we can do as drivers to make a difference, potentially, but right now it's very early days, very hard to know where we are for now just trying to optimise the car.

"[The power unit] feels good. But it's very hard to tell - the main thing is reliability at the moment.

"We have done a good amount of laps today and the only issues we had today were safety precautions, not actual issues. But hard to know where we're at compared to everyone else."