Pool Four leaders Harlequins fell to their first Heineken Cup defeat of the season in appalling conditions at Ravenhill.

With the driving wind and rain forcing the evacuation of the temporary stand, and some players having to be wrapped in blankets once the game had ended, Ulster - already unable to make the quarter-finals - built up an early lead and defended it efficiently in worsening conditions.

The visitors did narrow Ulster's 16-0 half-time lead to 16-10, thanks to a penalty try, but heroic defending from Matt Williams' men saw them end the game with a breakaway try from skipper Rory Best.

Facing into the gale and horizontal rain, Harlequins made the worst possible start by conceding a try in the first minute after they had knocked on from a steepling kick-off.

From the scrum Ulster moved the ball right and Paddy Wallace fed Darren Cave who simply glided through the non-existent Quins defence for an easy score which Ian Humphreys converted.

The Ulster outside-half then added a penalty after nine minutes before Nick Evans could not get a grip of the howling wind and pushed a fairly straightforward effort wide.

After man-of-the-match Stephen Ferris had cut down Mike Ross with a huge tackle, Humphreys stepped up and nailed his second penalty from just short of the halfway line to give Ulster a 10-0 lead.

His third penalty duly arrived just before the break, shortly after Danny Care had been sin-binned for kicking the ball away, and Ulster turned around with a massive task ahead of them to keep Quins and the conditions at bay.

Evans immediately got Quins off the mark and the visitors then began to besiege Ulster in their own 22.

After several attempts to cross the line, a huge scrum and effort from Irish prop Mike Ross saw Isaac Boss dive into the scrum resulting in an inevitable penalty try which saw Evans add the extras.

From there it looked certain that Quins would push on to win, but Ulster's pressure defence and the woeful conditions saw the visitors' game plan come unstuck.

After turning down two potentially kickable penalties, substitute Chris Malone's first action was to go for goal in the 72nd minute. He pushed the kick wide and that was as good as it got for Quins.

With the last eight minutes seeing uncontested scrums due to Tom Court having to leave the field injured, Ulster upped their game.

A Paul Steinmetz dropout relieved the suffocating pressure for Ulster and then, with four minutes remaining, David Pollock charged down a Malone kick and poked it through with substitute Nigel Brady also leading the charge.

Brady and winger Tom Williams overran the rolling ball over the line and Best got the touchdown to give Ulster their second win in Pool Four ahead of next week's final game at Stade Francais.