The Pole is pushing for a return to F1 six-and-a-half years after he injured his right arm in a rallying crash.

However, he faces competition for the Williams seat alongside Lance Stroll from Paul di Resta, Felipe Massa and Pascal Wehrlein.

Williams has put together a dedicated testing programme in its 2014-spec car, used by Stroll for circuit familiarisation, to see if he is fit enough to return next season.

Kubica completed a full day's worth of running in a white-liveried FW36 in the behind-closed-doors test at a cloudy Silverstone on Wednesday.

Williams said the running was "successful" but is keen to keep results of the test under wraps and therefore did not release any details about the amount of laps covered or laptimes set.

The team plans to run him again at the Hungaroring - where Kubica drove a 2017-spec Renault for a day during in-season testing in August.

It is believed the test will take place next week, ahead of the United States Grand Prix at Austin, with reserve driver di Resta getting one day of running the following day.

Williams is in no rush to make a decision, given it has the most coveted seat left on the 2018 grid, but it is believed the outfit would like to be able to make an announcement before the end of the season.