Lotus is grappling with a major problem less than one month before the start of the 2012 season.

The former Renault team has sensationally abandoned this week's Barcelona test and returned to its Enstone headquarters, after an unspecified chassis fault was detected on the new E20 car on Tuesday.

"We'll put the right measures in place and we will be able to fix the problem before next week," said technical director James Allison.

But Gary Anderson, formerly the designer of Jordan and Jaguar cars, believes Lotus is playing down the problem.

"You would have thought they could have fixed it overnight -- chassis are made of carbon fibre composite so you'd be looking to bond new strengthening parts on," he told the BBC.

"In my experience, if you can't do it in 24 hours, I don't think you can do it at all. I wouldn't be surprised if this meant they needed a new chassis, which would make it touch and go for the first race."

Germany's Auto Motor und Sport thinks Lotus did not experience the fundamental problem at Jerez last week because the Circuit de Catalunya is a much more demanding circuit.

Meanwhile, Red Bull's world champion Sebastian Vettel was Tuesday's fastest as the second of three group tests began, but he admitted Lewis Hamilton - in the new McLaren - was also impressive.

"Ferrari is a bit of a secret at this stage and nobody knows whether they are really good or really bad," admitted the German.