Williams have confirmed that they will test both Robert Kubica and Paul di Resta in order to evaluate both drivers for a potential 2018 driver seat.
But the team have stressed that while Lance Stroll will continue to drive for them next season, the youngster could yet be part of an unchanged line-up alongside Felipe Massa.
It's believed that Kubica, out of F1 since 2011 when he suffered career-changing arm injuries, and Di Resta, Williams' current reserve driver, will run on consecutive days at the Hungaroring later this month in a 2014 car.
"We will test those two drivers," said Williams technical chief Paddy Lowe. "We won't give away any information around what we do within those tests, that's a private matter for us and I would stress that doesn't mean that they're the only drivers under consideration. As I said earlier, we're considering quite a large range of which they are only two possibilities."
Lowe also clarified that Massa, who has publicly questioned the value of the Di Resta-Kubica test, was still in contention to stay on for a fifth season.
"Felipe is very much in the frame, very high on our list of possibilities but we owe it to ourselves to take a look around and see what could be the best option for the team going forwards," Lowe said. "That will be a partner to Lance.
"All options are under consideration. You've probably seen a number of names that are floating around that we're looking at but honestly the range is almost unlimited. We will consider all ideas. We're not in a super hurry to do so and we'll just make sure we land the best line-up we can."
Pascal Wehrlein is also understood to be a contender for any vacancy which emerges although the German youngster's age may count against him due to the requirements of Williams' principal sponsors, Martini.
"We're always ambitious to move forwards, as all teams are, and I think we recognise that the driver or drivers are a very key element in the team," Lowe added. "Clearly you need a quick car as well, but the driver is probably the single most important factor in the end to take the package forward and score the points and indeed win races. I've seen that in the past, what a difference drivers can make to the race proposition, so it's a very important decision."