LONDON, Aug 26 (Reuters) - Mercedes ⁠will be allowed engine upgrades, despite leading both Formula One championships by a considerable margin, ⁠after the governing FIA concluded that Red Bull -- fourth overall and without a win in ‌2026 -- had the strongest power unit.

The FIA published findings of its Additional Development and Upgrade Opportunities (ADUO) mechanism on Wednesday and said Mercedes could make one additional engine homologation in 2026 and another in 2027, with corresponding cost cap and dyno testing increases.

ADUO ​grants Formula One power unit manufacturers additional upgrades if the performance ⁠of their internal combustion engine (ICE) falls at ⁠least 2% below the sport's best-performing one across defined review periods.

The first review period ended after the ⁠Canadian ‌Grand Prix in May and the second one after the Hungarian Grand Prix in late July.

The sport has entered a new engine era this season and ADUO is designed to prevent one ⁠manufacturer from establishing a significant advantage without rivals being able to ​respond.

Mercedes were found to be ‌in the 2-4% window for the first period while Audi, Ferrari and Honda were above ⁠4% and granted two ​additional homologations in 2026 and two next year.

Mercedes provide four teams with engines, Ferrari three and Audi and Honda one each.

Red Bull were not mentioned, confirming their engine -- also used by sister team Racing Bulls -- was considered the ⁠benchmark despite this season being their first with their own in-house ​power unit in partnership with Ford.

After 12 rounds, Mercedes lead Ferrari by 87 points with Red Bull 239 points off the lead. Champions McLaren, third overall and winners of the last two races, also use Mercedes ⁠engines.

The ADUO ratings had already leaked out after the first period, causing controversy given Mercedes' dominance in winning the first six races of the season including five in a row for championship leader Kimi Antonelli.

Red Bull had sought clarification in June, concerned that more competitive rivals were being given a development advantage.

"We were ​all a little bit surprised with that news," Red Bull's four-time world ⁠champion Max Verstappen said at the time.

No additional homologations were granted for the second period.

Nikolas Tombazis, the FIA's ​single-seater director, said the analysis had been "thorough and exhaustive" and ‌the governing body was confident in the findings.

"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 added.

(Reporting by Alan BaldwinEditing by Toby Davis)