McLaren racing director Eric Boullier has confirmed the team's full 2016 spec Formula 1 car will not hit the track until the opening grand prix in Australia.
The team has added numerous parts to the car over the past few days of the final pre-season test at Barcelona, which Autosport understands included front and rear wings, sidepods and floor.
But with McLaren being "a little bit behind" on what Boullier has described as its manufacturing and production programme, some parts have yet to be seen.
"We are not running the full 2016 spec here," said Boullier.
"It means we will get to Australia with more bits on the car, which unfortunately have not been signed off here, but it is what it is."
Boullier has declared being "too ambitious" as the reason behind the delays with the parts, adding "There's nothing wrong. When you stretch too far, sometimes it doesn't work.
"But everything is fine. We missed by one day, unfortunately. That's it. It happens sometimes.
"We are obviously trying to stretch, to be on the edge. We wanted to give the maximum time to the aerodynamicists to develop the package.
"Then you have to compress the lead-time on the production, and if something happens you are behind."
Boullier is optimistic about the parts to come given the data being fed back, adding: "There is always a discrepancy somewhere, but it is more or less correlating quite well."
The MP4-31 has at least proven reliable in testing for the most part, compared to its predecessor's abysmal form in testing a year ago, and subsequent season, after the arrival of power unit partner Honda.
Boullier is satisfied progress has been made.
"We have been able to run at least most of the days, which is very good news," he said.
"In terms of chassis, it is important we could go through all of the systems, which we couldn't do last year.
"For us, it's a good achievement, I think now we have a car ready for the start of the season, so I am definitely more confident and I think there will be more to come.
"The base looks OK and there will be much more performance coming in the coming weeks."
As for Honda, new motorsport boss Yusuke Hasegawa has confirmed the power unit has been homologated, and the spec on show this week will be the one that runs in Albert Park.
Echoing Boullier's comments on the chassis, Hasegawa said the engine is not yet at its full potential, but there is at least a more solid platform on which to build in comparison to a year ago.
"We have to improve our engine through the season, but as a starting point, I think it's a very good starting point," said Hasegawa.