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MELBOURNE, March 15 (Reuters) - Red Bull team boss Christian Horner has laid the blame for his team's terrible start to the Formula One season with their Renault engine, describing it as "undriveable".
Once the most powerful team in the sport, Red Bull won the constructors' championship four years in a row before finishing second to Mercedes last year.
And while Mercedes continued their dominance with world champion Lewis Hamilton and Nico Rosberg finishing first and second in Sunday's Australian opener, Red Bull fell even further behind.
Daniel Ricciardo finished sixth in his home race while his new Russian team mate Daniil Kvyat withdrew before the start with a gearbox failure.
"It's been a very tough weekend for Renault," Horner told reporters.
"The engine is just quite undriveable. You can see and hear that from the comments the drivers are making...you're not able to drive the car properly."
Horner has become increasingly critical of the French engine maker after they experienced problems adapting to the new V6 turbo hybrid era last year.
He first raised his concerns with Renault in 2012 and is frustrated that things seem to be getting worse rather than better.
"It's frustrating that we're effectively even further back than we were in Abu Dhabi (last November) in both power and driveability," he said.
"Ferrari have made a good step. Renault, at this stage, appear to have made a retrograde step."
Horner estimated that Renault's power unit was 100 horsepower down on Mercedes and he said it was paramount that the gap was closed.
"It's important after this weekend that we re-group with Renault and try and offer our support where we can, because they're obviously in a bit of a mess at the moment," he said.
To compound Red Bull's problems, Ricciardo used up one of his four power units during practice, leaving him with three for the rest of the season.
"The positives are that we finished," Ricciardo said. "The reliability was there for one car at least.
"I feel for Dani (Kyvat) but we'll turn it around and try and learn from the laps I did today and move on.
"We know where a lot of (the problems) lie but there's some more as well which was unexpected. Definitely behind but it is what it is and we've got to try and make the best of it.
"I'm sure we'll get there but it could be a little while." (Reporting by Julian Linden, editing by Alan Baldwin)