Gloucester gave one of the best performances of the season to bring Harlequins' winning run to an end with a 29-7 thrashing at the Stoop.
Quins had won their last five Premiership fixtures to move up to third in the Premiership but were destroyed by fourth placed Gloucester, who moved to within two points of their opponents.
Jason Woodward, Ruan Ackermann, Tom Marshall and Danny Cipriani and scored tries for Gloucester with Billy Twelvetrees adding three conversions and a penalty.
Nathan Earle scored a try for Quins which Marcus Smith converted.
Harlequins made the better start but with their first attack, the visitors took an eighth-minute lead.
Marshall made a strong run down the right flank to put Quins on the back foot and after a number of recycles, Woodward bumped off a one-to-one tackle from Mike Brown to score.
Twelvetrees converted before Gloucester stunned their opponents with a superb second try. From inside his own half, Ollie Thorley sprinted 40 metres before vital contributions from Marshall and Franco Mostert and a long pass from Cipriani provided Ackermann with an easy run-in.
Marshall then blotted his copybook by being sin-binned for a deliberate off-side to prevent a likely Quins' score but the hosts could not capitalise on the wing's absence.
Elementary errors were a major problem from Quins, which stopped them from building up any momentum, despite playing with the advantage of a stiff wind.
Kicks going out on the full and careless handling were frequent features of their play and it came as no surprise when a Twelvetrees penalty extended Gloucester lead to 17-0 at half-time.
The home side made two changes at the interval with Marcus Smith and Joe Marler replacing James Lang and Lewis Boyce with Smith making an immediate impact by breaking out of defence and then kicking ahead before nailing Thorley with a strong tackle.
Rising star Alex Dombrandt and scrum-half Dave Lewis were also withdrawn as Quins needed something miraculous to get back into contention and their cause was not helped when Joe Marchant picked up a yellow card for a high tackle on Thorley.
Gloucester took immediate advantage when a flowing three-quarter move ended with Marshall flying over in the corner.
Marchant returned from the sin-bin but it made no difference as Cipriani scored their bonus point try with a conversion from Twelvetrees putting Gloucester out of sight.
Leading lights Cipriani and Thorley then departed as Quins' fans streamed from the ground to ponder a surprisingly inept performance from their team with Earle's try three minutes from time a mere consolation.
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