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McLaren do not expect the major upgrade they are introducing to their car at the next race in Spain to catapult them back to the front.

The team have struggled in the first four races, with a best result of fifth place for Jenson Button in China.

Button said he was "pragmatic" about the upgrade's potential impact.

He said: "There'll be elements that work, elements that perhaps work in a different way than anticipated and elements that don't work. That's life."

McLaren ended last season with the fastest car, with Lewis Hamilton and Button winning the last two races of 2012.

But they chose to make major design changes for 2013 while most other teams evolved their previous cars, and its aerodynamics have not worked as expected.

The car has been suffering from a lack of downforce and poor ride and McLaren were so desperate to make some progress after the first two races that they introduced some new parts for the third race in China without them going through the team's normal rigorous simulation process.

Button said he hoped that running the car again on the Circuit de Catalunya, where two of the three pre-season tests took place, would give McLaren fresh insight into the car.

"It's been difficult for the team to make consistent progress in the first four races," he said, "but I think returning to a circuit where we undertook two of the pre-season tests will give us a useful benchmark for our progress so far.

"There's been a lot of talk about the importance of next weekend's upgrades but, as with every upgrade, they're simply part of the series of continuous improvements that are made across the season."

He added: "Of course, I'm hopeful that it'll move us a step closer to the destination."

Team principal Martin Whitmarsh said only that the team were "pushing ahead to develop MP4-28 and will be hoping for a productive weekend that will allow us gather a useful data set for the races ahead".