Fernando Alonso says a redesigned Ferrari car must give the team a major step forward at the next race in Spain if he is to fight for the title.
The double champion, whose car has been uncompetitive so far this year, has this week tested revised rear bodywork, which is part of a major upgrade.
"We were on average 0.8-0.9 seconds off the top guys in the first four races," he said.
"We need to reduce this immediately if we want to fight for the championship."
Alonso added: "Barcelona is one of the most important steps we have to do. We must do it."
The new bodywork, which also features a revised exhaust position, is just the first step in a series of upgrades due on the Ferrari.
New front and rear wings and a revised diffuser are also due for the Spanish Grand Prix on 13-15 May before further changes at the subsequent races in Canada and Valencia. Monaco is between Spain and Canada.
Alonso has won widespread praise for some superb driving that has enabled him to be fifth in the championship, only 10 points off Red Bull's leader Sebastian Vettel, after the first four races, despite having an uncompetitive car.
"The upgrades Ferrari introduced in Mugello are definitely an improvement. It looks like a reasonable step forward - you could be looking at 0.2 seconds a lap, possibly more.
The exhausts and sidepods are much more conventional now.
There is nothing too exciting about the exhaust exits but the 'coke-bottle' is a lot tidier than it was. There is still some upper rear bodywork sticking out into that area but not anywhere near as much as before.
There is no exhaust interference, and while they're struggling with the car it is better to get rid of that for now and go in search of the bigger gains they need.
Before, they were chasing the smaller gains from the exhaust but losing much more by compromising overall performance.
One reason there is still some bodywork there is that Ferrari are also using that area as an exit for the air from the radiators."
He has qualified no higher than ninth, having struggled to get into the final top-10 shoot-out in all races, yet he won in mixed conditions in Malaysia and scored points in Australia, China and Bahrain.
He said after testing the upgrade at Mugello in Italy: "We know there is no magic button that in Barcelona we will be on pole position because everybody will improve their cars as well, but we need to reduce this gap.
"Barcelona has to be the first step, in Canada the second step, Valencia third and then be close to them.
"We cannot be fighting for Q3 if we want to be fighting for the championship.
"We are curious to see how much we can improve the car and how much the others improve theirs and hopefully our step is bigger."
The new Ferrari design makes the rear bodywork, in front of and between the rear wheels, much more tightly waisted than before.
It has the benefit of less bodywork obstructing the critical 'coke-bottle' area at the rear, where the bodywork sweeps inwards towards the back of the car.
It also moves the exhaust exits further into the centre of the car, abandoning attempts to gain an aerodynamic benefit from blowing them at the area where the rear wheels meet the floor in an attempt to 'seal' the diffuser and increase downforce.
All teams have been exploring different exhaust positions in an attempt to recover performance lost to the ban on exhaust-blown diffusers - last year's must-have technology, where teams boosted downforce by blowing exhaust gases along the rear floor of the cars even when the driver was off the throttle.
Alonso said the evidence of the first four races, in which cars with very different exhaust designs have been competitive at different times, suggested continuing to chase this area was a waste of time.
"It is not important any more which [exhaust] position you choose and how you manage the position," he said, adding: "I think this year we cannot lose even five minutes on this and there are bigger areas on the car we need to improve and we try to do it."
Alonso set the third fastest time on the final day of the Mugello test, despite an excursion off track in the morning which broke the car's front wing.
He ended up 0.328 seconds slower than pace-setter Romain Grosjean of Lotus. They were split by Red Bull's Sebastian Vettel.
Grosjean's fastest time was one minute 21.035 seconds - 0.232secs quicker than Vettel.
McLaren were the only team not to use their race drivers at the test, preferring to employ test drivers Gary Paffett and Oliver Turvey, instead of Lewis Hamilton and Jenson Button.
Red Bull have an upgrade package for the Spanish Grand Prix, but in Mugello were focusing on set-up work with an aim to better understand the 2012 Pirelli tyres.
Lotus also focused on tyre assessment, as well as trying out some aerodynamic and suspension changes to the car.
Grosjean's time was set in the afternoon, when conditions were windier than the morning, when Vettel and Alonso set their fastest laps, although Alonso also got close to his best time later in the day.
Lotus were pleased with the performance of the car. Trackside operations director Alan Permane said they were "flying - it's really good to be able to do 21.0 in the windier afternoon".