Williams looks set to replace the retiring Felipe Massa with Canadian rookie Lance Stroll for 2017.
Earlier, Jenson Button was a leading candidate to replace Massa next year, but the Briton announced at Monza that he is taking a sabbatical and can return only with McLaren in 2018.
Speed Week reports that "various sources" are now reporting that Williams’ next lineup will be Valtteri Bottas alongside Stroll, who will be just 18 if he makes his grand prix debut in Melbourne next March.
Stroll, whose father is the fashion billionaire Lawrence Stroll, is currently a Williams development driver and the championship leader of European F3.
A F1 team boss said at Monza: "It’s a done deal, provided that he gets a super license from the FIA."
Marc Surer, a former F1 driver working as a pundit for German television, commented: "For me it is clear — Williams needs money and the Stroll family has it.
"Luckily, young Stroll also has a lot of talent."