The left front suspension failed as soon as the Toro Rosso driver hit the brakes and he couldn't slow before running into the gravel trap, flipping, and hitting the barriers.
Hartley was speeding down the Wellington Straight, reportedly at 200 miles per hour when a failure in front-left suspension and steering caused the crash.
Hamilton, a five-time victor at Silverstone, posted a best lap of 1:26.722 seconds around the 5.891-kilometre course but Kimi Raikkonen of Ferrari was just 0.093 seconds behind.
Vettel completed only eight laps during the session and did not take part in the final minutes due to a "nuisance on his neck", as confirmed by Ferrari.
After this point, the Directorate has suspended the session, and the track arrived ambulance.
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While another said: "Honestly no need for this" and another said: "Can't they just enjoy their success without wrecking the place?"
Aside from Leclerc, only Haas pair Kevin Magnussen and Romain Grosjean were able to get within two seconds of the time achieved by pacesetter Hamilton, as they classified eighth and ninth respectively.
Despite the head-on impact, Hartley was able to climb out of the vehicle unscathed and was cleared and released after being checked in the medical center, leaving him fit to take part in qualifying if possible.
A Toro Rosso spokesperson told Formula1.com the team completed a full front suspension change on Gasly's auto ahead of the one-hour session that will decide Sunday's grid.
Gasly's early time, prior to Hartley's failure, left him 19th on the leaderboard.
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