M-Sport has decided against running its latest-specification Ford Fiesta RS WRCs in Mexico next week.
The British team will test the parts further before their competitive debut instead.
The facelifted Ford had been expected to run in Leon for the first time, but team principal Malcolm Wilson says there is more work to do before the parts can be fitted.
Wilson told AUTOSPORT: "Unfortunately, the proposed facelift is not as simple as just adding a new front bumper - as some might imagine - and our designers and engineers have been working hard to ensure that everything is just as it should be.
"We've been running the facelift on our test cars all season and recently discovered some issues we had not anticipated.
"We're eager to see the new-look Fiesta on the stages as soon as possible, but it won't be added to the competition cars until we are completely happy."
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