McLaren-Mercedes duo Jenson Button and Lewis Hamilton have admitted that the build-up to the F1 2011 World Championship campaign with the team's bold new MP4-26 has been far from plain sailing, with a lack of downforce and persistent reliability woes hindering preparations and leaving them needing to do 'twice as much work' between now and the Melbourne curtain-raiser as chief rivals Red Bull Racing and Ferrari.

Whilst Red Bull's RB7 ominously looks to be every bit as potent as its world-beating RB6 predecessor, and Ferrari's F150th Italia has been metronomic in its consistency thus far, McLaren's new baby has been rather less impressive, battling against repeated reliability gremlins - with a lack of spare parts during testing at Jerez in southern Spain last month limiting Hamilton to a mere 36 laps on one of the days, and further frustrations in Barcelona. And even when it has run solidly, the nervous-looking MP4-26 has hardly set the ground trembling.

There is now just one test remaining before the F1 travelling circus heads Down Under for the season-opening Australian Grand Prix at the end of the month, and as things stand, McLaren scarcely appear to be in much better shape than in either 2009 or 2010, having begun both of the past two campaigns on the back foot and playing catch-up - a weakness that arguably cost the Woking-based outfit last year's crown.

The MP4-26 was the last of the front-running contenders to take to the track - missing the Valencia outing at the beginning of February - and Button has conceded that whilst he knew the minute he stepped into the Brawn BGP 001 two years ago that it would be a force to be reckoned with, he has yet to experience the same feeling with his new toy.

"When I first jumped in it, the first reaction was not 'wow, we're going to blow everyone away', no," the 31-year-old told Reuters and The Times. "I don't think anyone would have felt that, though, because it's got a lot less downforce and the [Pirelli] tyres are working very differently to the previous [Bridgestone] tyres. There's nothing that really scares me about the car in a negative way, so there's a lot we can improve with this car through general set-up work.

"It's tricky because we've had some issues in testing in terms of getting parts to the circuit and a couple of reliability issues, so we've not done as much running as we would have liked - that has hurt our set-up work, which is disappointing. When you're limited on parts and you have reliability issues, you don't get the testing done - and when it comes to developing a new car, it is essential to do a lot of kilometres.

"It's been bitty. We have had some good runs, but the problem is we have not done enough to see our pace compared with others - we haven't got everything together yet, so we don't really know where we are compared to the competition.

"We've improved, but not enough. The next test [in Barcelona next week] is really important, and hopefully everything's going to run sweet and we get a lot of laps in and we can improve the base that we have. If we went racing now, we wouldn't be quite ready - but there's a lot still to extract from this car that we haven't, because we just haven't had time to do it."

"I have definitely not completed the laps that I expected to," added the man who triumphed in Melbourne in both 2009 and 2010, speaking to Italian magazine Autosprint. "This car has a lot of potential, but if the development work is late, there may be a gap in the first races of the championship.

"Barcelona will be important for putting everything together, but I think it will be very difficult to get an exact idea of the hierarchy. I've heard rumours that the Red Bull is six tenths faster than the Ferrari, and also that Ferrari are one second faster than everyone on race pace! I think that to get an overall picture, we will have to wait until the first race..."

Hamilton has echoed his team-mate and fellow world champion's fears that the problems with the MP4-26 may not be resolved in time for Australia and that he and Button might begin proceedings off the pace - as he underlined the potential consequences of failing to hit the ground running when the lights go out.

"Of course we'd love to have had more mileage," the 26-year-old told Reuters. "One day we did only 38 laps. It's nothing major, but we have had mechanical issues at the back of the car and that has really hampered us. We have picked up and definitely improved - we got 107 laps [on one day] at the last test - but we are behind on mileage and we can't catch it up in the testing left.

"The car handles really nice - better than last year's car - but it just doesn't have as much downforce, so that's where it feels weak. Hopefully we've got some more downforce coming onto the car before the first race. I think we've good things coming in the pipeline, but it is always difficult to catch people. Wherever we are right now, we will have to do twice as much work with our update package for the first race because so will Ferrari and Red Bull."