Williams chief Pat Symonds has admitted that the British team is no longer Formula 1's third force.

In 2014 and again last year, Williams finished the constructors' championship in third place, behind Red Bull in 2014 and Ferrari last year as engine supplier Mercedes dominated in the new power unit era.

"Either one of them (Red Bull or Ferrari) was in front of us or the other, but at the moment it's both" he told Germany's Auto Motor und Sport.

"When you've already come up with lots of clever things, it is incredibly difficult with our budget to keep up with teams that spend twice as much."

Williams are third in the 2016 team rankings after the first four grands prix of the season.