Fernando Alonso will only be able to do "limited mileage" on Friday, after McLaren-Honda confirmed that a re-designed component will not arrive in Barcelona until Saturday.

McLaren-Honda had to curtail running early on Thursday, with Jenson Button managing just three runs and 21 laps before a broken seal on the MGU-K caused a problem that brought his MP4-30 to a halt.

"We had to stop running this morning after discovering a faulty seal on the MGU-K. That faulty part then caused an issue that requires full replacement of the power-unit - and the time needed to make those changes effectively ends today's programme," explained Honda's chief motorsport officer Yasuhisa Arai.

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"We'll need to run a re-designed component to avoid a potential repeat of the problem. Unfortunately, we won't be in possession of that updated part until Saturday, which means that we may face the same problem when we run again tomorrow.

"Such issues are all part of testing, but we'll make the best changes we can overnight, and will plan for a productive day of running - with Fernando behind the wheel - tomorrow.

"Hopefully, with a bespoke solution ready for Saturday, both drivers will have an opportunity to get some useful laps under their belts, with Jenson driving on Saturday, and Fernando concluding the test on Sunday."

The outfit also added that it will now revise Friday's programme as a result.

"With the prospect of limited mileage, the team will look at undertaking some time-intensive bigger set-up changes in a bid to maximise learning about the car's balance - so expect lengthy breaks between runs throughout the day," it warned.