Stockdale, whose touchdown helped Ireland beat world champions New Zealand last month, struck in the first half at Parc-y-Scarlets to underpin a 25-24 bonus-point victory.

His fellow wing Henry Speight, centre Will Addison and number eight Marcell Coetzee also scored, with scrum-half John Cooney kicking a penalty and conversion to leave Ulster level on points with Pool Four pace-setters Racing 92.

But Scarlets - European semi-finalists last season - are facing an early exit following a third-successive defeat as two Steff Evans tries, a late Dan Davis try, a Rhys Patchell penalty and two conversions plus Dan Jones' conversion, proved in vain.

The teams meet again in Belfast next Friday, and all the momentum is with Ulster, whose remaining Champions Cup fixtures also include a potentially-pivotal home clash against Racing next month.

Scarlets welcomed back Wales forwards Samson Lee and James Davies, who both missed the autumn Tests because of injuries, but full-back Leigh Halfpenny remained sidelined as he continued his recovery from concussion suffered against Australia four weeks ago.

Ulster, meanwhile, included Stockdale, Addison and their Ireland Test colleagues Rory Best and Iain Henderson, with Best captaining the team.

Patchell and Cooney exchanged early penalties, while Ulster stretched the Scarlets defence through a strong run by Speight, and it proved a warning for what was to come just five minutes later.

Ulster attacked from a scrum inside their own half, with Addison easily rounding Scarlets and Wales centre Jonathan Davies, before possession found its way to Stockdale, who showed his class in blistering fashion.

Stockdale - scorer of 12 tries in 14 Ireland appearances - brushed aside the challenges of wing Evans and scrum-half Gareth Davies to complete a sweeping 60-metre move in style, and although Cooney's conversion attempt drifted wide on a blustery night in west Wales, Ulster were off and running.

Scarlets lock Lewis Rawlins went off for assessment after being on the receiving end of an Ulster challenge that referee Alexandre Ruiz reviewed, but took no action other than to award the home side a penalty.

But the Scarlets forwards then moved up a gear, driving deep into Ulster's 22 and setting up an imposing attacking platform, from which Evans prospered to claim a close-range try that Patchell converted.

Scarlets' lead only lasted eight minutes, though, as a poor Patchell clearance kick allowed Ulster the chance to run possession back and - after some patient build-up work - Speight crossed for his team's second try and secured a 13-10 interval advantage.

Ulster extended their lead 10 minutes after the restart when Cooney found Addison with a defence-splitting pass, and the centre finished effortlessly before Cooney's conversion opened up a commanding lead.

It was the cue for Scarlets to start putting width on their game, and Jonathan Davies was instrumental through his powerful carrying and support play before Best secured a penalty for his team and Ulster relieved the pressure.

But Evans set up the final quarter when he sprinted over for his second try, with Patchell's conversion cutting the gap to 20-17, yet Ulster responded immediately as Coetzee went over in the corner.

And Coetzee's touchdown proved the telling score, with the Scarlets left reeling despite a stoppage-time Davis try that Jones converted, leaving them looking down and out in Europe.

- Press Association