McLaren are working to produce a car fast enough for world champion Hamilton
McLaren say they face a race against time to match the pace of their rivals ahead of the opening Grand Prix of the 2009 Formula One season on 29 March.
World champion Lewis Hamilton's car has performed poorly in pre-season testing.
Team boss Martin Whitmarsh said: "We are working hard to resolve a performance shortfall."
Norbert Haug, engine partner Mercedes' motorsport boss, added: "We are definitely not fast enough and not competitive enough to aim for wins."
During this week's test in Barcelona, Hamilton and his team-mate Heikki Kovalainen were well adrift of rivals Ferrari, BMW Sauber, Renault, Toyota and the cars of the new Brawn team, who have only been officially in existence for a week.
Hamilton finished bottom of the timesheets on Wednesday after damaging his car's front wing and nose in a crash and was almost two seconds slower than Rubens Barrichello's fastest time for Brawn GP on Thursday.
When asked if the MP4-24 was fast enough, Whitmarsh conceded: "Not at the moment - and certainly not by our team's extremely high standards.
"Next week we will be testing at Jerez [in Spain], which many of our rivals will not be.
"We aim to continue to develop the car, and the result should be measurable on the stopwatch.
"Will MP4-24 be as quick as we want it to be by 29 March? Perhaps not.
"Will it be quicker than it has been this week at the Barcelona test? Yes."
The MP4-24 is running with an updated aerodynamic package in Barcelona but is still struggling
The 2009 season, which kicks off with the Australian Grand Prix, sees the sport's biggest rule changes for 25 years.
The new regulations, designed to increase overtaking, have reduced the cars' aerodynamic downforce, reintroduced untreaded "slick" tyres and given teams the option to employ a kinetic energy recovery system (Kers).
McLaren's poor performances over the winter have led to suggestions that the team have not effectively interpreted the rule changes.
McLaren chairman Ron Dennis conceded this week that the development of the car was a little behind schedule.
The team did not begin consistently running with the car's 2009 rear wing - which rules demand must be taller and narrower than the 2008 version - until Barcelona this week after using last year's model during earlier testing in Portugal and Jerez.
Whitmarsh confirmed the car's new aerodynamic package is at the root of the team's problems.
"Initial testing of the MP4-24, which first ran with an interim aero package, went in accordance with our early developmental expectations," he said.
I have never seen McLaren so far behind
"This week the car has run in Barcelona with an updated aero package and a performance shortfall has been identified
"Our Mercedes-Benz engine is strong so the MP4-24's performance shortfall is clearly chassis-centric.
"Ron, Norbert and I have enormous faith in the ability of our engineers and we are working as hard as they are to make the MP4-24 competitive enough to win Grands Prix."
But Ferrari's Felipe Massa, who lost the 2008 championship to Hamilton by a single point, said he was surprised by McLaren's apparent lack of pace in Barcelona.
"I have never seen McLaren so far behind," Massa told Italy's Gazzetta dello Sport newspaper.
"However, they have another test to see what is not working.
"They are a team which can improve from one day to another, so you always need to respect them."
Whitmarsh and Haug are adamant that they will find the performance to challenge their oldest rivals Ferrari, and the rest of the field, over the 17-race season.
"Expect us to fight back even if it takes some time," warned Haug.
Whitmarsh added: "I am utterly certain that, once we have ironed out the issues the MP4-24 currently faces, we will win as a team again."