Renault's power unit in the season-opening Australian GP is believed to have been a massive 100bhp down on Mercedes's field-leading package.

Under fire and under power, the French manufacturer have come under fresh public criticism from Red Bull in the wake of the former world champions’ dire start to the season in Melbourne when they only finished sixth and were lapped by both Mercedes drivers.

Red Bull chief Christian Horner hit out at Renault’s lack of progress after the race, describing their V6 unit as “undriveable” and claiming the firm had even managed to go backwards since the final winter test at the start of the month.

“We're probably 100bhp [brake horsepower] down on Mercedes at the moment,” a downbeat Horner revealed.

Given Renault had been optimistic of closing the gap to Mercedes in 2015, the engine maker has expressed frustration that the improvements it insists were made during the winter weren’t carried through to the race engines used in Australia.

“We know that we made genuine progress over the winter but we could not show it here and in fact we would even seem to have moved backwards,” Renault Sport managing director Cyril Abiteboul said.

Red Bull slipped behind Ferrari at the first race

“Given the pace at which we conducted our development programme towards the last few weeks of the winter, there may not be lots to change to be able to access these improvements.”

Remi Taffin, Renault’s direction of operations, added: “It’s related to the maps, or the way the power unit is configured, so while it’s definitely not an easy fix, it does not require a complete redesign.”

Writing in his Sky Sports F1 column, expert analyst Mark Hughes suggested that had Renault’s simulations been correct for the first race then Red Bull would likely have been between Mercedes and Ferrari in the pecking order.

“With a year’s worth of development it was expected that Renault could reduce that [40bhp] deficit, but the version of the engine delivered for Melbourne, which was expected to have 50bhp more than that used in Barcelona testing, actually had less power and much worse driveability,” Hughes wrote.

“In Melbourne it was as much as 100bhp down on the Mercedes, which has gained an extra 50bhp since 2014. Had Renault delivered what its simulation had promised, the Red Bull’s pace would likely have been somewhere between Mercedes and the much improved Ferrari and the competitive picture would not have looked so dire.”

Somewhat embarrassingly, there ended up being little difference between Red Bull and Toro Rosso in Melbourne – although Horner denied the company’s junior team was having fewer problems with the Renault power unit.

“It [engine underperformance] masks so many things. Corner entry, corner exit, degradation, slip control of the tyre, you’re not able to drive the car properly,” he explained.

“You then start moving your brake balance around to try and compensate and you are so far away from optimum. They are struggling the same amount, it’s just having a more dramatic effect on our car.”

Red Bull also experienced unreliability during testing

Renault’s power deficit is likely to be a particular handicap at next week’s Malaysia GP with the Sepang circuit featuring back-to-back long straights at the end of the lap.

“We’ve got a lot to do and it’s going to be a busy few weeks,” Horner added. "It’s disappointing but we will get our heads down and work hard at it. We’ll push and work with Renault as best we can.”

Renault’s latest engines struggles have come amid further speculation about their future in F1, with suggestions continuing that they may look to restablish their own team. Reports in recent days have said that representatives from the company have visited Toro Rosso, Force India and Lotus factories.

Speaking at the final winter test, Taffin said that Renault's engine department would simply adapt to the company’s wider F1 strategy were things to change.

“My view is I rely on my boss to deal with these things. He told me we have to achieve the best power unit for this year which we do and if we have to achieve the best car in the future then we will do the same thing,” he said.