McLaren opted not to run its latest-specification front wing in qualifying for the Spanish Grand Prix because it did not have the chance to check it would pass the FIA's bodywork flexibility tests.
A pair of new front wings reached Barcelona overnight, but ahead of Saturday morning practice the decision was made not to risk running them without checking.
"It was very tight," said team principal Martin Whitmarsh when asked why the new front wing design was not used.
"We were unable to be confident that they complied legally and we took the decision that unless you are confident in the compliance of the wings then you shouldn't use them."
AUTOSPORT's guide to the Spain tech updates
McLaren had hoped to test the flexibility of the parts on the FIA test apparatus but did not get the opportunity to do so before free practice.
While the wings complied with the regulations when tested in-house, the team did not want to leave itself open to disqualification should its tolerances have been incorrect.
"You will see people queuing down at what is known as the 'bridge of doom' and it's the tests that go on there that determine whether the car is legal or not.
"We and other teams test parts before they are here, the tolerances of stiffness and dimensions is very tight.
"Everyone does their own test but everyone tests on critical issues on the FIA equipment.
"If you can't do that, given the tolerances, you have got inherent risk."
Whitmarsh did not blame the FIA for not being able to test the wing, accepting that the apparatus did not have to be available to McLaren when it wanted to use it.
"The hope was that we would have the ability to get onto the rig but in fairness to the FIA, they have got no obligation to make it available to use early on a Saturday morning so there is no criticism of them - that was the chance that we took.
"Without that check, it wasn't prudent to go forward."
PROGRESS... BUT NOT ENOUGH
McLaren's major upgrade package did improve the car, with Sergio Perez turning in its best qualifying performance of the year with seventh fastest time in Q2, but Whitmarsh admitted that it could have been better.
He pointed to the improved correlation between factory simulations and track performance as one major positive.
"The simulation before the start of the season was over-optimistic and we had poor correlation but often when you look back at it and are really honest with yourself [you realise] there was some will to believe data and to interpret data in a more positive way than it really [should have been].
"The simulation coming here was more modest in expectation.
"We hoped that we would find more performance than the simulation and if you want to take some positives out of the weekend, there is a better correlation.
"But clearly we haven't made the step forward that we need or want to make."
McLaren has further upgrades planned for the upcoming Monaco and Canadian Grands Prix, building on the Spanish GP package, which includes a new front wing first run on Friday, a new rear wing designs and modified rear suspension uprights amid a raft of changes.