Red Bull have confirmed reports that a scheduled engine upgrade has been delayed by supplier Renault.
With the struggling former champions preparing a 'B' car for Barcelona, it was said that Red Bull could join the fight at the front by June also thanks to a Renault engine upgrade.
However, it was reported this week that the upgrade has been postponed "indefinitely" for reliability reasons.
Helmut Marko, a top Red Bull official, told Auto Motor und Sport: "As long as Renault does not have reliability under control, there is no further development.
"This is a setback for us."
At the same time, rumours are swirling that Renault are actually accelerating development on a single car - the works machine raced by Nico Hulkenberg.
"The fuel may be different but the hardware is the same, Renault assures us," said Marko. "It would be impossible to build two different engines anyway.
"You only get this impression (that Hulkenberg has a faster engine) in qualifying, but one reason could be that Hulkenberg is a frighteningly good driver in qualifying."
For now, Red Bull are putting their eggs in the 'B car' basket, with simulations promising a huge step forward of a full second per lap.
"We know all the numbers but now I want to see it on the stopwatch in Barcelona," Marko said.
Red Bull are currently third in the constructors' standings.
