Formula 1: Mercedes, Red Bull & Williams unveil cars at testing

Nico Rosberg drives the new Mercedes at pre-season testing in Spain

The first day of pre-season testing for the new Formula 1 season has started at Spain's Jerez track with three leading teams unveiling their cars.

World champions Mercedes, runners-up Red Bull and third-placed Williams showed off their new designs in the pit lane before beginning preparation work.

Eight teams are in Jerez discovering whether the months of design work on revising their machines has paid off.

Force India have missed the test because their 2015 car is late.

The big secret was the Red Bull, as Mercedes and Williams had already released pictures or images of their challengers.

Most of the changes to the Red Bull car are "under the skin", chief engineer Rob Marshall says

It was launched in a temporary camouflage livery, similar to those used by road-car manufacturers when they are testing new models.

What could be seen, however, was that the car features a nose design that combines elements of last year's Ferrari with the "bump" on the tip featured on the new Williams.

"There are some changes that affect the shape of the front of the car but beyond that most of the changes are under the skin," Red Bull chief engineering officer Rob Marshall said.

"There won't be a lot that's visible to the naked eye but a lot of hard work has gone into the bits that are hidden."

The Mercedes, like the other cars, is an evolution of last year's dominant machine, but it features notable differences, including a narrower and longer nose and narrower rear bodywork.

"The key factor from our perspective is avoiding complacency. Expectations are now high and a lot of assumptions are being made about our potential this season," Mercedes executive director (technical) Paddy Lowe said.

"Internally, however, we are fully aware that you can never afford to stand still in any sport - particularly F1.

Williams hope their new FW37 car will help "continue their move back towards the front of the grid"

"We have been pushing harder than ever to find areas for performance gains. But at the same time, we must ensure we are moving in the correct direction.

"Some changes will be more visually obvious, of course, but the devil is in the detail. Beneath the covers there have been a raft of developments on both chassis and power-unit - all aimed at creating a car that is safer, more efficient, more reliable and ultimately faster."

Sir Frank Williams said his team's target, after leaping from ninth to third last season, was "to continue their move back towards the front of the grid".

The new FW37 bears a strong resemblance to its predecessor, but has an unusual nose design to meet the new rules aimed at increasing safety, with a bump protruding from a square section at the front.

Elsewhere, Sebastian Vettel was the first man out of the pit lane as he began work for his new team, Ferrari.

And McLaren had a delayed start as they begin their much-anticipated era with new engine partner Honda. Fernando Alonso will drive on Sunday before alternating with Jenson Button for the rest of the test.

Button wrote on his Twitter account: "Don't expect too much from us this test as it'll be mostly systems checks", adding that McLaren-Honda faced a "long road" but had "massive potential".