St Helens continued their relentless march towards the double with an outstanding victory against arch rivals Leeds that took them through to a third successive Carnegie Challenge Cup final.

A 16th successive League and Cup win was never in doubt once the evergreen Sean Long sent Chris Flannery over for the opening try just seven minutes into a physical and fast-paced semi-final at Huddersfield's Galpharm Stadium.

Saints will now be overwhelming favourites to make it a third successive triumph in coach Daniel Anderson's final season at the club when they take on Wakefield or Hull at Wembley on August 30.

Leeds staged a second-half rally after trailing 18-2 but they were easily second best on a day when little went right for them.

Already without New Zealand Test full-back Brent Webb, who failed to shake off a back injury, the defending champions lost influential scrum-half Rob Burrow after only 17 minutes when he came off much the worse from an attempted tackle on Lee Gilmour.

Stand-off Danny McGuire struggled from the early stages with a leg knock and centre Clinton Toopi, who had earlier been placed on report for an alleged spear tackle on Paul Clough, hobbled off with a knee injury five minutes before the break.

Saints scored their second try while Toopi was writhing in agony on the ground but there was little doubt that Saints deserved their 12-2 interval lead.

Apart from the opening moments when second rower Ali Lauitiiti briefly threatened to cause havoc with his offloads, Leeds were a clear second best in all areas of the game.

Second rower Gareth Ellis was impressive in a hard-working defence but the Rhinos could not match Saints' attacking prowess.

Skipper Keiron Cunningham, who will make an eighth Cup final appearance, gave them a magnificent lead with his trademark surges from dummy half and, once softened up, his replacement James Roby provided new dangers for the Yorkshiremen.

Leeds stand-in full-back Lee Smith and rookie winger Ryan Hall were forced to withstand a barrage of intelligent kicks from Jon Wilkin and half-backs Long and Leon Pryce and the only surprise was that Saints led only 6-2 for the majority of the first half.

It was Long who created the first try for loose forward Flannery, one of only two Saints players not involved in last year's final, with a cleverly-delayed pass.

Leeds captain Kevin Sinfield pulled two points back with a penalty, which temporarily relieved the pressure, but the Leeds defence lived dangerously until Pryce took Long's pass to jink his way over for a second try.

Long's second conversion made it 12-2 and, when Wilkin finished off a superb break by left winger Francis Meli eight minutes into the second half, it looked all over.

But, whether it was the sun in their eyes or their earlier efforts taking its toll, St Helens began to make mistakes and Leeds were quick to capitalise.

Lauitiiti plunged over for the Rhinos' first try on 54 minutes and four minutes later winger Scott Donald took Keith Senior's pass to squeeze over at the corner.

Sinfield added one conversion to cut the gap to six points and Leeds fans among the 19,842 crowd sensed a momentum shift, especially when Burrow returned to the action on the hour.

But Saints were able to regroup and Long calmed their nerves with a penalty before Bryn Hargreaves took a short pass from fellow front rower James Graham, who won the man-of-the-match award, to score the clinching try, to which Long added a fifth goal from as many attempts.

Leeds had the consolation of providing the final try, courtesy of a blockbusting run from prop Kylie Leuluai, but when Sinfield made a hash of the simple conversion it summed up their day.