That is the admission of Red Bull chiefs Christian Horner and Dr Helmut Marko, although they are confident works supplier Renault is on track with a better engine for 2015.

It has now been tested at Jerez and Barcelona, and lead driver Daniel Ricciardo revealed last week the 2016 power unit "definitely has some more ponies".

"It does have more power," team boss Horner confirmed to Germany's Auto Motor und Sport, "but it is not evenly distributed.

"There is still a problem with driveability," he added.

The possibility

However, his Austrian colleague Marko said some of that problem can be solved through "software changes".

"Then we will have caught up a large part of Mercedes lead," he predicted.