Leinster ended rivals Ulster's perfect start to the United Rugby Championship season with a 20-13 victory at a rain-soaked Kingspan Stadium.

John Andrew responded for Ulster before Aaron Sexton had a try disallowed for a knock-on as the hosts fell short.

Leinster top the URC table alongside the Bulls, who beat Connacht on Friday.

Ulster, who grabbed a losing bonus-point, slip to third place after losing their first game of the season to their inter-provincial rivals.

Rain impacts first-half showing

As expected in a game between the URC's top two sides heading into the weekend, despite a number of absentees for Emerging Ireland's tour of South Africa, the match started with a frenetic four-minute spell without a stoppage as Ulster began to press the visiting 22, albeit without penetrating the Leinster defence.

However, in clinical fashion in Leinster's first attack of the game, back row Baird powered his way over in the corner after some fast and flowing Leinster hands in the eighth minute, and Ross Byrne was on hand to add the conversion.

Dan McFarland's team came again, but once more struggled to break down a hearty Leinster defence, although John Cooney's well-taken penalty on 18 minutes was consolation for their efforts.

Leinster responded three minutes later when Sheehan drove over from a maul for Leinster's second try, however it resulted from a penalty which was one of Ulster's own making.

Wing Aaron Sexton took a quick throw under pressure and referee Andrew Brace spotted an infringement when the ball went to ground.

Byrne added the conversion, and to add insult to injury Ulster lost influential hooker Rob Herring to injury in the midst of the score.

When the rain arrived, any real quality disappeared. Both teams became error-strewn as the downpour worsened and saturated players, fans and pitch alike.

However, Leinster seemed to have better control in the conditions and a Byrne penalty five minutes before half-time moved the scoreboard along and gave the visitors a 14-point advantage at the interval.

Ulster fight back as rain relents

Ulster lost Jacob Stockdale at the break after the wing ended the half limping with an ankle injury and Byrne kicked Leinster further clear with his second penalty five minutes after the restart as the rain continued to teem down around south Belfast.

Leinster were camped deep in Ulster's half in the opening stages after the restart but failed to make any further inroads on the scoreboard as the white wall of shirts held firm when it looked like the visitors would capitalise.

The rain finally relented midway through the half, and with it the Leinster pressure.

Ulster were finally able to mount an attack of their own, and their pressing was rewarded when Jimmy O'Brien was sent to the bin before Andrew drove over for the try from the resulting line-out.

Cooney added the conversion to bring Ulster to within 10 points of their Irish rivals as Leinster called upon Ireland captain Johnny Sexton to steady the ship.

Ulster thought they had pulled within touching distance with 11 minutes to play when winger Sexton raced over after collecting Michael Lowry's pass, however his attempt was ruled out by the TMO after it was adjudged Sexton lost the ball forward in contact with the recovering Charlie Ngatai as he went to ground the ball.

Cooney did bring Ulster to within a converted score with another well-taken penalty with seven minutes to play.

However, Leinster showed their years of title-winning experience to run the clock down and hold off Ulster to claim victory in Belfast.

Ulster: Lowry; Sexton, Marshall, McCloskey, Stockdale; Burns, Cooney; Warwick, Herring, O'Toole, O'Connor (capt), Treadwell; Matty Rea, Marcus Rea, Timoney.

Replacements: Andrew, O'Sullivan, Moore, Carter, Jones, Shanahan, Curtis, Moxham.

Leinster: O'Brien; Larmour, Ringrose (capt), Henshaw, Kearney; Byrne, McGrath; Porter, Sheehan, Ala'alatoa, Molony, Jenkins, Baird, Van der Flier, Conan.

Replacements: McKee, E Byrne, Abdaladze, J Ryan, Connors, McCarthy, Sexton, Ngatai.