Red Bull remains the only manufacturer unable to upgrade its Formula 1 power unit - after the FIA announced no change to its ranking of engines.

Under F1’s catch-up mechanism known as ADUO (Additional Development and Upgrade Opportunities), manufacturers that were more than 2% adrift of the benchmark are allowed extra development scope.

This includes an increased cost cap allowance, more dyno hours, plus the ability to introduce new homologations in-season.

If a manufacturer is more than 2% adrift they get one upgrade this season and one the next, while anyone more than 4% off gets two upgrades this year and two in 2027.

Following the first ADUO period that ended at the Canadian Grand Prix, the FIA’s measurements had concluded that Red Bull had the best internal combustion engine, with Mercedes 2% behind and everyone else 4% off.

That decision triggered controversy, with Red Bull adamant that its data showed it was never that far clear of championship leaders Mercedes – and it opened dialogue with the governing body to review the findings.

But following the conclusion of that analysis, plus the FIA’s findings from the second ADUO period that ran from Monaco to Hungary, there has been no change in the FIA's view.

In a statement issued on Wednesday, the FIA said that its awarding of ADUO allowances was unchanged – meaning Red Bull has to stick with what it has got for now and will not get any extra development freedom.

The FIA did not reveal any details of what the findings from the second ADUO period were but, with things being unchanged from the first period, it means that no manufacturer was deemed to have improved enough to overtake Red Bull by at least 2%.

Under ADUO, the extra allowances are not cumulative - so once they have been awarded once, no further opportunities will be given for the same threshold.

The situation means that unless one of Red Bull’s rivals delivers an upgrade to overtake the Red Bull engine by over 2% by the time of the third ADUO review period, which runs from the Dutch GP until the USA GP, then Red Bull will be stuck in being unable to improve.

While Red Bull has questioned the findings of the FIA’s first ADUO assessment, the governing body has defended the quality of its analysis.

Nikolas Tombazis, the FIA’s single seater director, said: “The FIA has undertaken thorough and exhaustive analysis as part of our ADUO review process.

“With the release of the findings of period 2, we are confident that, based on the quality of the data, its analysis is incredibly robust, giving us a high level of confidence in our findings."

Tombazis also reiterated that the FIA cannot release specific numbers: “We also have to respect the confidentiality and intellectual property of the teams and their power unit manufacturers in disseminating any details from the findings.”

The controversy over the assessment that has put Red Bull on top has triggered talk about the need to revise the ADUO process.

One element that has come under the spotlight is that ADUO is based purely on the performance of the internal combustion engine element and ignores other performance factors like the batteries.

It is widely accepted in the F1 pitlane that Mercedes has the best package overall because its energy management is more efficient - but it has been awarded ADUO because its ICE is not the best.

Tombazis says the FIA is open to making changes to the way ADUO operates, but that this would require support from teams and manufacturers.

"This is the first year of ADUO so naturally there are areas which we may evolve over time, in discussion with all the PU manufacturers,” he said.