Sebastian Vettel is switching to a different Red Bull chassis this weekend in a desperate bid to salvage his title defence after being outpaced by rivals Mercedes so far this year.

The four-time world champion said it was a move aimed at helping the team to understand why the German driver had languished behind his rivals, including new Red Bull team-mate, Australian Daniel Ricciardo.

“I think we concluded after China, where we were quite a little bit behind, to change the chassis,” Vettel said.

“It is not a new chassis, it is an old one we used in testing in the winter and we have some experience with it.

“It is just to try everything we can.”

Vettel explained that he was making the move for this weekend’s Spanish Grand Prix in case car problems contributed to his recent struggles.

Although he finished on the podium in Malaysia, he was only sixth and fifth in the following races in Bahrain and China — and had to move aside for team-mate Ricciardo on both occasions.

Notably, when Red Bull confirmed Vettel would use a new chassis in Spain, the team suggested it was a scheduled move.

But Vettel said the decision was made after Shanghai as part of an effort to ensure every possible cause of his troubles was investigated.

He told reporters: “It is not unusual to change chassis generally, we decided to change back to an older chassis just to make sure that nothing is wrong.”

Meanwhile, Jenson Button believes his McLaren team are “miles behind” pace-setting rivals Mercedes and have no chance of catching them this weekend.

He thinks it will be “months” before any other team has a chance to match the pace and performance of front-runners Nico Rosberg and Lewis Hamilton.

“We’re miles behind the Mercedes, but we’re in a good little scrap with five other teams,” Button told reporters.

“We’re not worried about later in the season. We feel we can develop faster than most teams.

“We’re adding updates to the car all the time, some bigger than others, but I think for any team to find a second is a big ask over the next couple of races.”

Drivers championship leader Rosberg and his team-mate Hamilton have dominated the opening four races of 2014 for Mercedes.

McLaren, who took second and third in the season-opening Australian Grand Prix thanks to strong drives from Kevin Magnussen and Button, have slipped back since that promising start.

Button finished a lap behind Briton Hamilton, the 2008 champion, at the Chinese Grand Prix and Dane Magnussen has also struggled.

Fernando Alonso put Ferrari on the podium in China but ruled out thoughts of a repeat performance in his home race.

“We will do our best,” said the two-time world champion who won his and Ferrari’s last race a year ago in front of his fellow-Spaniards.

He agreed with Button that it was possible to catch Mercedes before they were beyond reach in the title race.

“China was a combination of things, and good luck to get that podium. Here in Barcelona is a good point to check how competitive we are, a circuit with different characteristics again and a circuit that will give us an answer or some of the parameters where we need to find some answers.”