Red Bull Racing team principal Christian Horner has admitted his team hasn't completed the build on its 2015 F1 challenger just days before it is set to hit the track in Jerez for pre-season testing.

The team, which is seeking to reclaim its F1 crown after being comprehensively beaten by the dominant Mercedes in 2014, has spent the winter attempting to bring itself up to speed, but has left the build of its Renault-powered car very late.

With just four days before the car will be rolled out for the first day of testing in Spain, though Horner acknowledges the team is cutting it fine, he insists it will be ready on time.

The car is not ready yet, he said. The car is flat out into car build at the moment, so it is probably the shortest production and assembly in ten seasons. It is unbelievable what the factory has turned around so far [but there is] till plenty to do before Sunday.

Despite the tight turnaround, Horner is confident Red Bull will avoid a repeat of last year's disastrous opening test, which saw it complete only a handful of laps due to persistent technical gremlins

This one can't be any worse than that, he continued. I think we managed four laps over four days and the car would either stop of track or catch fire. I think it has been a far more positive winter compared to last year.

We know where the benchmark is and what we need to achieve. As the team continues to evolve and grow, I think it has been the most impressive winter we have had as a team in terms of everything coming together. From that perspective it has been a positive winter.

Red Bull Racing heads into 2015 with renewed optimism, led by a revised driver line-up which sees Daniel Ricciardo replace Ferrari-bound Sebastian Vettel as team leader and Daniil Kvyat step up from Toro Rosso.

Furthermore, Red Bull and Toro Rosso will become the sole focus of engine suppliers Renault, with Horner saying the French firm has been busy chasing an 'aggressive' development process to bring its much maligned hybrid power unit up to speed.

This year strategically you can upgrade during the year. Renault are taking an aggressive development approach to this year and strategically we just need to decide when to use those tokens in the four engines you have available to you.

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Red Bull 2015 F1 car not ready yet