Adrian Newey says Renault and Ferrari face "a very difficult challenge" trying to catch up with Mercedes in 2015.

The Mercedes power unit was dominant under the new regulations in 2014, with Lewis Hamilton winning the championship after a season-long battle with team-mate Nico Rosberg. While Daniel Ricciardo won three races for Red Bull, Williams - powered by Mercedes - often had the second quickest car and Newey admits it's going to be difficult for any of the other engine suppliers to catch up.

"Can Renault and Ferrari get to Mercedes' level? It's going to be a very difficult challenge - and of course we don't know where Honda are going to come in," Newey told Sky Sports News HQ.

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"With a slightly weaker engine but a superior chassis you can still win and vice versa. We have a reasonably even balance between engine and chassis - which, in my view, is how it should be."

And Newey said he felt the Mercedes power unit was the only reason Williams was able to outpace Red Bull at times during 2014.

"Clearly, last year was engine-dominated and we saw that with Williams springing forward. Obviously Williams did a very job but I think what really made the difference for them was the engine."