Toto Wolff admits the decision to introduce an updated Mercedes power unit ahead of the Italian Grand Prix was a 'risky call' as Nico Rosberg saw his hopes of winning this year's Formula 1 world title take a huge blow due to reliability problems.

Mercedes spent all seven of its remaining in-season development 'tokens' by giving Rosberg and Lewis Hamilton the chance to use an upgraded power unit in Monza as the team sought an opportunity to get a head-start on its programme for the 2016 season.

However, a fundamental issue with the cooling system led to a leak that 'contaminated' Rosberg's new power unit, forcing the team to revert him back to his 'old' specification engine for qualifying and the race.

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Saying it was always going to 'compromise' his chances against Hamilton, Rosberg was still in contention for a second place finish when the ageing unit - competing in its sixth race - cried enough with just two laps of the race remaining. His and Mercedes' first DNF of 2015, Rosberg has subsequently dropped 53 points behind Hamilton in the overall standings with seven races remaining.

With the benefit of hindsight, Wolff admits bringing the upgraded engine - which he insists is to test fuel efficiency - to Monza was a 'risky' one, but insists it will be beneficial going forward, regardless of Rosberg's misfortune.

"We brought that phase four engine because we want to understand if the direction of development was the right one," he said. "It was a bit of a risky call. We saw what happened to Nico and that was the result of that engine.

"The reliability runs were on quite a high mileage but they weren't finished yet, so in hindsight we lost a car and Nico lost valuable point. But this is a very competitive championship. It is going to be one next year and the earlier you can understand which direction you need to go the better it is.

"The engine was on its sixth race, so it was very high on miles and we didn't get any warnings no. We gave it a little push to try to catch up Sebastian and this is when it decided to call it a day.

Significantly, Mercedes fear the engine could be damaged beyond repair, which puts him closer to the cusp of a potential penalty should he be forced to change two more engines before the end of the season.

"We don't know yet whether it is terminally damaged, and to make the point - it wasn't an engine specific failure we had. It was a leak in the cooling system which led to the engine to fail."