England prop Mako Vunipola made a powerful return to action following nine weeks out injured -- but Aviva Premiership champions Saracens suffered a second successive league defeat.

Vunipola, who had been sidelined by a knee ligament problem since mid-December, proved his fitness in front of watching England head coach Eddie Jones by playing 70 minutes at Kingsholm as Saracens went down 31-23.

But while Vunipola could now be involved when England resume their Six Nations campaign against Italy on Sunday week, there was another injury worry for Scotland boss Vern Cotter as Saracens' Sean Maitland went off early in the second-half after appearing to suffer a rib injury.

Cotter has already lost skipper Greig Laidlaw and number eight Josh Strauss for Scotland's remaining Six Nations games, and wing Maitland could now give cause for concern ahead of next Saturday's Murrayfield appointment with Wales.

Saracens, meanwhile, were undone by an impressive Gloucester performance that saw full-back Tom Marshall, lock Jeremy Thrush and hooker Richard Hibbard score tries, while fly-half Billy Burns kicked 13 points and centre Billy Twelvetrees booted a late penalty.

Burns, though, was carried off injured following a 10-minute delay in the second-half while he received treatment, having gone down after he attempted to make a tackle.

It was the first time since May 2015 for Saracens to experience back-to-back defeats -- they lost to Worcester last weekend -- but they could have few complaints, despite hooker Schalk Brits' early try and a Will Fraser touchdown, two Alex Lozowski penalties and two conversions, plus centre Marcelo Bosch's long-range penalty strike.

Gloucester showed five changes from the side beaten comfortably by Leicester last weekend, with centre Matt Scott and number eight Ben Morgan among those called up, while there was also a start for Ross Moriarty following his outstanding display in Wales colours against Six Nations opponents England.

And Gloucester were quickest out of the traps as a 45-metre Burns penalty put them in front before Saracens a brilliant response that had Vunipola at its attacking hub.

Saracens went through several attacking phases, slowly but surely pushing Gloucester back, before Vunipola off-loaded superbly out of a tackle and scrum-half Richard Wigglesworth sent Brits over.

Lozowski converted before a long-range Bosch penalty opened up a 10-3 advantage, yet Gloucester kept up the early point a minute scoring rate when they capitalised on Twelvetrees' break, with scrum-half Willi Heinz delivering the scoring pass to Marshall.

Burns' conversion levelled the contest, and after a Lozowski penalty restored Saracens' lead, Gloucester were left ruing a missed opportunity following more strong work by an impressive Heinz.

This time, he kicked cleverly into space, and wing David Halaifonua gathered possession before crossing the opposition line, but Wigglesworth got his boot under the ball and Halaifonua could not ground it.

It was an escape for Saracens, and although the visitors temporary relieved pressure and saw Lozowski kick another penalty, Gloucester were not to be denied an interval advantage as two further Burns penalties during a dominant five-minute spell made it 16-13 at the break.

Gloucester extended their advantage just nine minutes into the second period, with Saracens proving architects of their own downfall as Bosch and Fraser both made defensive errors and Thrush enjoyed a clear run to the line for a try that Burns converted.

A Lozowski penalty set up an intriguing final quarter, and Saracens drew level when Fraser was driven over for a try barely 60 seconds after Glouccester prop Paul Doran-Jones was yellow carded for pulling down a maul, and Lozowski's conversion made it 23-23.

But Saracens fell away during the closing stages, and after Twelvetrees struck a penalty after taking over kicking duties from Burns, Wales international Hibbard crashed over from close range to seal a famous Gloucester win.