Williams Chief Technical Officer Pat Symonds has admitted that he is undecided to as whether the team will use an upgraded Mercedes' engine during 2015.

In the recent Italian Grand Prix the Merced team only ran o0ne of the new units after suffering reliability issues with one. As a result Symonds is cautious in switching to the new engine before it is fully understood. Apart from this the current specification of Mercedes engine that the Williams team is using has been producing results.

"To be honest I don't know," Symonds told Sky Sports when asked if Williams would use the new Mercedes engine this year.

"There is obviously some risk analysis to be undertaken to know whether to use the new power unit or not. I think we are incredibly satisfied with what we have got, we have got the best power unit in Formula 1 and Mercedes allow us to use it very, very aggressively. So we feel it is a very reliable and a very powerful unit and that is great for us.

"We are competitive people and we always want the next best thing, but I think we will be guided by the works team by how they get on - remember they have only done one race with one car with that power unit and there will be a lot of inspection going on.

"What it is really good to see, though, is that they are not resting on their laurels, they are the dominant power unit, yet they are still pushing really hard for 2016."

"We have got a fourth engine to take, maybe somewhere around America, maybe Russia, we will see how it goes," Symonds added.

"But we want to take our guidance from Mercedes, we don't want to take risks. We will see what they think is the best engine for us."