Toto Wolff has explained why Mercedes isn't offering its most recent update to its customer teams, insisting it could ultimately prove more detrimental to their performance in the remaining four races.

Mercedes used all of its remaining tokens to update its power unit ahead of the Italian Grand Prix, though the engine has so far only been used by the factory Mercedes AMG team and is unlikely to be offered to Williams, Lotus and Force India before the year is out.

It is a move that irked Williams somewhat, with Rob Smedley contending that it would be beneficial to have more cars using the power unit as a way of developing it towards 2016.

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However, Wolff disagrees, saying the upgrade was primarily a means of getting a head-start on ensuring reliability - rather than performance -, a factor it doesn't want to compromise by stretching resources to rush out the upgrade for another team at this time.

"The engine is very much an R&D engine. We decided on trying out the direction that we believe could be beneficial for next year but we are not 100% sure. In order to do that exercise you can only concentrate part wise and logistically on the two engines.

"When we had the problem with Nico's engine [in Monza] we weren't even sure we had the spare parts to give him another engine. This is why we decided to do it. From phase three to phase four, the main thing it is not a performance step up. Of course people always say that engine must be much better, but that is not the case and it is just a development direction for next year.

"We can probably run it a bit harder for a bit longer, but it is not the miracle step up and we just can't supply it logistically."