he team said on Saturday the problem resulted from the way its recently-introduced rear wing was interacting with the floor, and has since decided to revert to the previous wing design as well as changing some other parts.
A change of specification after qualifying automatically results in a pitlane start.
The Williams drivers will line up ahead of Toro Rosso's Brendon Hartley, who did not take part in qualifying after a major crash in FP3, and has changed his chassis overnight.
Sirotkin and Stroll were due to start 18th and 19th respectively.
"We brought in some new pieces [on Friday] and we need to do more work," said technical chief Paddy Lowe after qualifying.
"We have an intermittent problem with the floor stall, related only to DRS. It doesn't recover well enough for the subsequent corner.
"We can only assume now that it's related to the new pieces even though we did a proper introduction and seemed to have a stable platform as we finished FP2 yesterday.
"The phenomenon is something we've never seen before on this car or indeed any other. We have the rear wing, our previous rear wing, which is absolutely fine in this aspect."